I think this question boils down to a balance between backwards compatibility and technological progress. You can strive to maintain the ultimate in backwards compatibility, and never change a lens mount. Some camera manufacturers have succeeded in that, such as Nikon and Pentax, to a large degree. However, what is the long term cost of progress there?
Canon made a very explicit decision back in the late 80's when they created the EF mount. Their existing mount did not provide the capabilities that they needed to support extremely fast AF, so they made the decision to ditch FD and create EF. Despite ruffling feathers, from a business and progress standpoint, it was the best decision they could have made. The EF mount allowed them to create very wide aperture, VERY fast AF lenses that otherwise were not possible with the FD mount, and that rocketed them to the top of the list for huge numbers of photographers. The benefits of the EF mount and fast AF with very wide lenses was enough to eventually cause droves of photographers to leave their previous brand [look in the 1990's section] (which was largely Nikon at the time, I believe) and move to Canon. That little fact is not as obvious today, since we have had EF for around 26 years now, and Nikon caught up to Canon's AF many, many years ago.
Sometimes the question is not as simple as simply maintaining backwards compatibility or "timelessness" (as the FD mount was once called.) Sometimes the benefits of progress far outweigh the benefits of longevity, and a change needs to be made. It should be noted that not every single Nikon lens works perfectly with moderns Nikon DSLR's. Older, purely manual F mount lenses should work great, however older electronic F-mount lenses have been noted to have problems with modern DSLR electronics. Canon has had similar problems with some of their very old EF lenses from the late 80's when used on modern DSLR's. Such cases are rare, but they do happen...and its just the basic cost of progress.
All of that said, the EF mount did not entirely make the FD mount moot and useless on modern Canon DSLR's. Adapter rings exist that allow you to use FD lenses on EF bodies. Some of them include a correcting lens (like a teleconverter) that preserves infinity focus, while others act like a very small extension tube, possibly eliminating infinity focus but allowing close or macro focus distances. Using an adapter, a very wide range of FD lenses can be used on modern EF camera bodies. The story does not stop there, either. Adapters exist for the EF mount that can adapt a variety of other brands of lenses as well, including the Nikon F mount, M42 lenses, and others. I believe that makes the EF mount one of the most versatile mounts available, and opens an incredibly large world of lens possibilities to Canon users that may not be as readily available for other brands or lens mounts.
As a Canon user myself, I have always enjoyed knowing that I could pick up a small adapter and use the manual focus FD lenses, such as the FD 500mm f/4.5 lens which seems to be a superb birding lens. I also like knowing that I can get a Nikon F mount adapter and use their excellent 14-24mm ultra-wide angle zoom lens. I have also recently discovered the M42 mount, and the extremely wide range of manual lenses available for that mount, which can also be adapted to Canon's EF mount.
I have to update this a bit. Before Canon's recent rounds of lens upgrades, I though optics were optics, and that they were already excellent. Canon's latest optical technology has proven me wrong, though. Particularly with their "Great White" telephoto and supertelephoto lenses (200mm f/2, 300mm f/2.8, 400mm f/2.8, 500mm f/4, 600mm f/4), the new line of Mark II lenses, as well as all of the recently released L-series zooms and primes, are MARKED improvements over the previous generations.
All of Canon's new lenses are faster (electronically/functionally, not aperture-wise), with better AF logic and significantly improved IS, lighter with more advanced materials like titanium and magnesium alloy barrels (some lenses have lost nearly four pounds of weight!), with significant feature upgrades (often for video, but those features can be useful for stills work as well, such as focus recall), and have SIGNIFICANT IQ improvements. Image quality from an MTF standpoint has improved considerably in the latest lenses, from the 0.8 to 0.9 range on most lenses, or ~0.9 - 0.95 on the previous generation of Great Whites, to nearly 1.0 at the center, and ~0.98-0.99 at the edge/corner (wide angles, like the 24-70, still suffer a bit more at the corner, however they are still improved over the previous generation). The use of fluorite elements reduces the number of lens elements necessary for any design, on top of greatly reducing CA, which improves aberrations across the board at wide open apertures. The use of nanocoatings on the most critical lens elements reduces flare to near zero, and increases transmission (nanocoatings, unlike multicoatings, avoid reflection almost entirely, rather than canceling reflections out...which still reduces transmission.) Image Stabilization has officially been doubled, from two stops to four stops. In practice, achieving at least five stops of improved hand-holdability, if not more, is easy. The introduction of Mode 3 IS with better, near-instant activation actuation supports the use of IS when the lens is mounted on a tripod, something that was usually not possible before (with the exception of the EF 800mm f/5.6 L, from which the new 4-stop IS system now used in all Canon telephoto lenses comes from.)
Simply put, Canon's newest generation of lenses offer nearly unparalleled IQ (maybe unparalleled in the DSLR world), with significant weight savings and considerable functional and performance improvements. They effectively invalidate the previous generation of many of Canon's lenses. Perhaps more so with the several thousand dollar "Great Whites", but also in the case of many staple lenses for other types of photographers, such as the 24-70 f/2.8 L.
Lens longevity is really a dependent thing. So long as a lens lineup doesn't receive a significant upgrade, older lenses will certainly last. In the face of a significant upgrade, such as Canon is currently performing to the bulk of their L-series lens lineup, old lenses quickly lose their luster in the face of all the benefits of upgrading. It has been nearly 15 years since Canon last updated their lens lineup significantly. With the considerable improvements in IQ and functionality, we can probably hope for at least another 15 years. However, I am not longer certain that any lens, even a $13,000 lens, will literally last a "lifetime" anymore. That would be on the order of fourty years. It is still intriguing to know I could pick up an 800mm FD lens and use it on my Canon body...but it is far less compelling now that I've used a new generation of lenses that blows my mind every time I see one of the photographs I've taken with it.
