Don't make the funeral arrangements yet.....
The 3.5" 12 gauge was created for two reasons.
The first, to pack more steel shot into a shell for greater effectiveness on waterfowl, especially bigger geese. Steel loads of the time came up rather short against geese if over 30 yards.
The second, to sell shotguns to folks with perfectly good shotguns at home.
Recent improvements in non toxic ammo reduced the "Need" for the 3.5" hull.
New stuff, like Hevishot 2s, is as good as lead or better. Better loads come out every year, and these are capable of being fired in 3 or 2 3/4" guns.
I do a fair amount of shotgunning, and so far I've not needed a 3.5" load.
Maybe half the shotguns here handle 3". None have 3.5" capabilities.
I have shot a few Supermags. With 2 3/4" loads, they work well......