as people are going bigger 37s-42s... sidewalls get taller too, with 15" rims. and 17'' wheels seem to be more available than 15's.
are 15's becoming obsolete?
looking to get a set, but dont want to be in an assbasket when i get new tires in the future and cant find any for 15" rims.
would like to get 33" or 35" tires

 

If you are not planing on going any larger then 35's. I am sure the 15" rime will stay around for that size. Larger brake diameter is the main driving factor that is pushing the rim size up. Smaller cars are still using smaller brakes and I am sure the 15" rim will stick around for some time in the future. There has been a market for 15's and that market will most likely be around for the future.

 

what about tire availability?
as larger rims become more common, will tires for 15" be more difficult to obtain?

 

got the same info from big'o and discount tires. bigger rotors need more room. and 15's aren't going anywhere, nor tires.