Blue - where do you live, and what do you drive? I find it hard to believe wheels are getting bent with a 25" tire on a 17" wheel. That's a crazy common size, even OEM.
I believe Blue lives in IL as do I. I can totally confirm that even a 16" wheel (with 205/50R16) is in danger on IL roads, let alone 17" or larger with thinner sidewalls.
As far as other thoughts:
15" tires in any size are becoming more scarce. It doesn't matter if it's for trucks, cars, etc. For sure, there will always be 15" tires available but the development of new and better tires for 15" wheels will be limited.
16" are the new 15". Still some tires available, but the are following the 15" as far as new development.
17" are what I believe will be the most common "smaller" wheels in the future. I would not feel afraid of 17" availability in the future. However it is clear that 18-20" wheels are here to stay.
20" at this time is about the practical limit to still maintain sidewall (for cars). I do not believe we will see a whole lot of development of bigger wheel sizes... but people said the same thing about "big" 16's in the 90's.
As far as the "best" wheel size, it depends. Generally, less sidewall means better handling. Generally, bigger wheels mean a heavier tire/wheel package, which means more unsprung weight (bad for handling and ride), and more rotation weight (bad for a low-powered vehicle). There have been tests with identical sets of wheels / tires on low-powered lighter vehicles, where the only difference was wheel size (e.g. 17 vs. 18 vs. 19) and the smaller wheels bested the larger ones.
As an example, for my Focus, I settled on 15" rims with snow tires for winter - plenty of sidewall to absorb bumps from the huge potholes that develop during winter. For three seasons I eschewed the typical 17" setup because of the low sidewall height and the poor roads here in IL, as well as the weight (though lightweight 17's are widely available). Instead I went with sticky 225/50R16's which are available as a niche tire for dual use racing and daily driving. They are slightly taller than stock but that is a benefit in my case. Additionally, the 16" wheels are generally lighter than 17's. Because of the construction of the tires, there is little to no tradeoff in handling even with the taller sidewall.
I am getting rid of our Volvo S40, but for that I had planned on 17's. 18's and 19's look awesome on them but the size is impractical due to our roads (the sidewalls would be 35 aspect ratio at best... very skinny). They would be bent in short order. Heck, I've had to fix 2 wheel bends on the stock 17's and I am a careful driver.