I have a couple model 12's but no 12ga nor did I use them for skeet or trap back in the day. The old man was a remington model 31 man then, wingmaster, superx1 etc so we didnot own too many
That being said I have 870's , 1100's and superx1's with 200,000 plus through them with no major issues other than rebuilding the 1100 every 2 weeks

so cannot see why a model 12 could not go forever.
I remember reading about a guy that was approaching 1 million rounds and there was a friend of my dad's that still has the sweetest trap and skeet versions I have ever seen and those would be well over 200,000 each with the only think I remember him replacing in the 45 years I seen him shoot was 2 firing pins.
Man could he shoot it also and it was not hard to know he was smoking targets with the dam bark out of those old Cutts on the end.
In fact thanks for reminding me he must be 80 plus now and has not shot in 5 years or so and would never sell but maybe time to ask again since he has no one left to leave them to that I know of
As far as repairs today I have no idea buit know even back in the day the boys would send theirs to a smith that worked at Epps at the time who was a crackerjack around them. Forget his name and he would probally be up there in age now also
Cheers