The answer to this question is that you have taken a wide ranging collection of terms and taken a very narrow range of meanings for them. If you use the meanings that many people do then the question fades away.
Your individual assumptions would be considered invalid in most cases my many photographers and, taken as a whole, would be considered invalid by almost all photographers.
professional use, - which you seem to think relates to only non moving well lit images where absolute optical quality is the main benchmark. This is not the case.
for static subjects some are, many are not. Cameras which cater more for static subjects are available, but are not the most popular.
at normal light, You assume that normal is the equaivelnt of studio lighting or bright sunlight. It's not. "Normal" spans 2 to 3 standard deviations either side of the mean. That's anywhere from awfully bright to gloomily dark. Fortunately.
best quality pictures you can get, You assume that quality is measured in some "ideal" terms of advert bright, crisp well focused imagery. It's not.
so you shouldn’t veer far off 100 ISO. Most photos would suffer from this. And it becomes increasingly unnecessary as equipment improves.
(high iso is) useful mostly in special situations this would require MOST of the images that I take and many that most take to be taken in special situations. Less than ideal light. Camera moving. Subject moving. ...
it is better to stick to the base ISO resolution. only for above defined meanings of better.
if in most situations ISO 100 is better, In MOST, it's not. In some it is.
why do camera manufacturers not restrict high ISO to specialties camera - Because they wish to provide cameras that people want, that do things that people want them to do and that people will buy. And because they are not fascists or doctrinaire ideologues who wish to force their perceptions of how photography "should" be on others
For regular use, what’s the point of an insanely high ISO? Insanely high ISO is useful for obtaining images that nothing else is fast enough for. Regular use in that context is any use that requires insanely high ISO. Only a few cameras have this ability, so far, alas. eg Sony A7SII - Oh yes!!! (409,600 max ISO).
Here's an A7S review to go on with "only" up to ISO 102,400.
[One handed out the far side window, while (sssh) driving, ... ]
Short: The photo below was taken very much "on the fly" at ISO1600, 1/3200s. At eg ISO 100 it would have been approximately impossible to achieve.
Longer: The best camera to use is the one that you have with you.
I have taken to carrying a not-so-new Sony NEX5T in my jacket pocket. My larger dinosaur may or may not accompany me - either on a 'rapid-strap' or in a shoulder bag. But, the 5T in a pocket is able to be carried almost anywhere without being obvious. I usually carry it set to ISO 1600 - and can rapidly alter that if desired. For everyday use with an acceptable level of noise and general quality at less than "pixel peeping" resolutions ISO1600 is acceptable. To my surprise.
For "driveby shootings" it allows photos like this. Nothing marvellous BUT the photo below is about 40% of a frame. ISO 1600, f/7.10, 1/3200S. Insane. If I was taking that with time to frame it, adjust the ISO, adjust the aperture, and ensure that composition and more was OK, as was after top quality - then I'd certainly not have used ISO 1600 - and not have need 1/3200s. BUT his was taken [while driving? - surely not !?] (naughty naughty) from the right side of a right-hand-drive van across the passengers seat and through the passengers window, one handed at perhaps 50 kph. At ISO 100 - not a show. If that does not fall within the range of "normal" for the photos you want to take, that's fine. For some, it does.
Sony NEX-5T, ISO 1600, f/7.10, 1/3200S. Driveby shooting.
Here's a report/review/love-fest on the king of insanity - the Sony A7SII
The Sony A7SII Review. The King of The Night gets updated. 24 pages (mainly photos) plus many reader comments.
ISO 50 - 409,600.
Example image at ISO128,000 1/20s, f4 - noisy as!!!.
But it's an image. Under 0.2 lux if I've done my sums right* (quite possibly not) - but very dim indeed regardless. The eye could not see detail at that light level.
- lux = 2.5 * 2^[(100 x f^2)/(ISO x t)]
t = exposure in seconds.
f = aperture eg 4 for f/4
1 lux = bright moonlight