The Sony A6000 is $500. Show me a full frame for that price, and I will accept your argument that even the body, let alone the lenses, are the same for a full frame dSLR as they are for an APS-C compact mirrorless. The only full frame camera I know of that matches even the A6300 or Fuji X-T10 in price is the A7(which is showing its age!), which is also mirrorless.
Compare APS-C dSLRs to APS-C mirrorless cameras, and full frame dSLRs to full frame mirrorless. Do that and I think you'll find the price and quality differences will become vanishingly small. You shouldn't ignore lens system, features, handling and ergonomics - these are really the biggest differences, and won't change much over time. Price and quality will come down to differences on brand and model more than fundamental aspects of mirrorless/dSLR systems as a whole.
Pick the brand, feeling, design system, ergonomics and philosophy(Sony is all about sensors, Fuji is more retro and widely better received ergonomics, just to mention a few) of the company you like best. It sounds silly, but those are the constants in this ever-changing world, and the cost of switching later goes up as you buy in.
Remember that the body is only a small part of the expense if you plan on getting deep into photography. Good luck!
Full frame DSLRs just as cheap as APS-C Compact Mirrorless
Ok, with several examples I can now make a comparison. The Nikon D600 and the Canon D6 were both launched in 2012.They are aging for sure. Even the A7 is getting old. This doesn't make them "bad", but it comes with it's own set of downsides. Things like autofocus, video, and ISO won't be as good as their current full frame counterparts. In many respects, much newer mirrorless cameras like the Fuji X-T10 and Sony a6300 will have caught up. In video, the a6300 is certainly better. In base ISO range, the Canon D6 is rated up to 25600 - exactly the same as the a6300. This is a huge blow to what I would consider a key advantage of full frame. Granted, this is mostly on paper, the full frame canon's outdated sensor is still probably better "in real life", but the differences are eroding.
What you end up with is all the weight of the full frame, with all the costs of the lenses (which do not come down in price dramatically over time like bodies do), but many of the advantages have been eroded by time. If you have never felt the difference, it is a very different feel from holding a little a6300 vs the heft of a good old full frame dSLR. Not something you casually walk around with. Now, especially in professional settings, that solidity, size, and heft screams professionalism, so that's something to take into consideration.
So, if video and weight are beans to you, and cost of full frame lenses has been taken into account, then the older cameras may be great. If the needs favor APS-C crop factor(so-called "reach"), or other newer features like 4k video, or you care about weight and bulk, APS-C is the way to go.
Ultimately, I'd go with the one that you see yourself using the most, even into the future. If you really, really want to go full frame, but can't afford a newer body yet, you might take the disadvantages of an older camera and then upgrade in the future(today's hot new cameras are the A7's and D6's of tomorrow), without having to sell your old lenses.