As there seems to be the odd Contax/Yashica system lens owner in this thread - I hope you guys are aware that there are
conversion kits available. The lenses themselves are still considered very good glass. I do have to wonder how they'd stack up against some more modern constructions
*cough* Samyang 135mm f/2 and 16mm f/2 *cough* but it's not like you couldn't have both. I do wish manufacturers would come out with some nice, reasonably compact fixed focal teles again, like the 135mm f/2.8s of yore. That 135mm f/2 is some first-rate glass but also about twice the size and weight.
I think it is obvious at this point that crop DSLRs may not have too much of a future. The long flange distance inherited from their full-frame film predecessors makes their wideangle lenses big, heavy and expensive at the very least. (Or small, but meh performers and still expensive, like the wide Pentax Limiteds. I still like the concept, but these days you'd rather have a cute lens on an equally cute mirrorless body.) Incidentally, the best ratio of sensor size to flange distance to date would have to be found in the brand new Nikon Z mirrorless system presented earlier this year - full frame and 16 mm, narrowly beating Leica SL @ 19 mm and Canon EOS R @ 20 mm. This is making crop systems with APS-C sensors and 44-48 mm look pretty sad. It's atually shorter than Micro-4/3 at 19.25 mm, a system with a sensor half the size.
Anyway, what we might still see is a square 36x36 mm image sensor - after all, lenses are circular. You could shoot full frame, full frame vertical, a square 30.6 mm format or anything in between.
Sigma's modern-day zooms are pretty solid as far as I'm concerned, I can't complain much about the 70-300 DG OS nor the 17-50 f/2.8 (I don't think it's
quite as good optically as the excellent Pentax 16-85, but it cost me a fraction on the used market and does fine for me). I think their reputation has gone up quite a bit since they came out with the excellent Art series lenses.
I think the main problems of fancy photography equipment these days are that either you can't be bothered to lug it along (size / weight), or you very much would like to take pictures but aren't allowed to.