The 39-2 is an iconic and historically significant 1st Gen S&W pistol. Production ceased in 1982. It was succeeded by the similar 2nd Gen Model 439.
The 915 is indeed a 3rd Gen. It was the first of the "value line" 3rd Gen pistols. The idea behind the value line were simply to reduce cost of manufacturing AND reduce the price. A 915 is basically a 5906 with these changes:
--black, not stainless
--aluminum frame, not stainless steel
--nondescript fixed rear sight instead of Novak
--single left side thumb safety/decock
The 915 occupies an interesting spot because S&W made them for about 2 years (and kicked out a SLEW of them in that time) before they decided that they hadn't "cheaped" them down enough. The other value line pistols that followed directly are not as nice as the 915.
It's little more than (seemingly typical) internet panic that parts are "hard to find" for these pistols. To put it bluntly -- it's nonsense.
Certainly, some parts are long gone directly from S&W in new condition. Places like Numrich and Brownell's is loaded with them.
S&W does not (for the most part) warranty these pistols, but like a lot of things... what will ACTUALLY happen if you need their help changes by the day, the rep on the phone and perhaps their mood.
Call them with a cracked frame and no, they cannot fix nor replace. But they may offer you some M&P at a crazy-low price in an attempt to honor the broken pistol.
I've got more than a dozen S&W semiauto pistols these days, they are my passion. Five 1st Gens, five 2nd Gens and a bunch of 3rd Gens.
Not one single "scarce" part am I worried about breaking. If and when I do need a part... thankfully, they made a gujillion of them and -MANY- of the parts work across many similar models.