Is my fear or concern unreasonable?
In my opinion, yes, yes it is.
You know when I read your post up top it came to mind I don't own a single firearm that is in current production. Nope not a one!
My small bunch of 3rd gens would be the youngest of my modest collection, or the ones most recently out of production so twenty years or thereabouts since many were made. I shoot them regularly and carry several without concern over parts breakage. If my pants are on there's one on my belt. And if not there's one or two within reach.
The oldest arms I own have been out of production for well over a century.
Military rifles dating from the Spanish American War up through the early Cold War years...Out of production.
Sporting arms from the 1890's up to 1955... Out of production.
There are plenty of parts out there for all these, even NOS parts for guns made in the nineteenth century.
Back to the 3rd gens... Tons of parts are out there. S&W shut down all the warranty repair stations in 2016 but those guys still have parts inventory and there are many other sources for parts.
I carry these guns because in my view they are superior anything currently produced for my purposes. The only current production pistol that has caught my attention is FNH's FNX-45... though not exactly well suited as a CCW
So I'll carry my 4506, 4516CCO, and Shorty45 mk2, and '1014'. They're all remarkably robust arms but in the extraordinarily unlikely event something breaks, I'll source the replacement part (assuming I don't already have one) and fix it. And... In the even more unlikely event I can't find the part I need I will bloody well make the thing and fix the gun anyway.
Life's too short that one must put up with cheap plastic rubbish throwaway guns with crappy triggers. Go get that 3913 or 3914 or perhaps a LS or NL variant or even a 3913TSW or perhaps a CS9 or 909 and enjoy.
Cheers
Bill
PS: Quality arms deserve quality leather goods. Personally I'm a big fan of Kramer's Horsehide, darned near as stiff as kydex but quiet and won't crack or break. I think those folks have blueguns for just about everything.