For the record I think the spell is probably fine as is for PCs. Just last session our monk got bit buy a bulette for about 35 damage. I'm sure she would appreciate taking only half damage and might be inclined to not use her bonus action to dodge or shadow step away but instead might take the hit...so yeah, I can see uses for it. I'm still very dubious that it is worth the 100gp cost though. And I do think the game is missing something by making defensive buffs better to be cast on allies. Our wizard literally hides around the corner during fights if he has a buff going on the paladin or monk...then the the earth elemental comes through the wall and he craps himself....
I think Stoneskin is fine. Concentration saves aren't that difficult when you're taking half damage, and most monsters don't have magic weapons. If the concentration requirement were removed, this spell would go back to being a must-have buff that every wizard would cast before every fight. I'd rather not go back to the "good old days" of flying, invisible, stoneskinned wizards running roughshod over every encounter.
Unless you are spending significant resources otherwise, concentration saves are difficult enough for something that is caused with every hit taken, no matter how small the hit is. That's caused by the minimum 10 save dc and by most casters not being proficient in them. It's not like you're sacrificing anything by attacking the wizard more - you just get to stop his spells as a free bonus on the path to killing him.
This seems to suggest that your casters are heavily prioritizing defense if they typically have 14+ dex and 16+ con, and the fighter version with heavy armor is swapping out the dex for a 15+ str.I haven't seen a lot of Fighter 1/Wizard X at my table (I did make one for my OotA platonic test party but that's about it). However, I have seen plenty of bards and wizards who are reasonably difficulty to damage (AC 16 + Shield) and have decent Con saves (+3 = 70% chance of success on Con saves).
This seems to suggest that your casters are heavily prioritizing defense if they typically have 14+ dex and 16+ con, and the fighter version with heavy armor is swapping out the dex for a 15+ str.
My opinion? It's a garbage spell, something I would never cast.
And for those saying that concentration isn't a big deal, I envy your ability to shrug off those saves. I've had terrible luck passing those saves, to the point that I retired the character because I was losing most of my spells after a single round; the minimum save is 10, and when you only have an 11 Con and no feats are allowed, it kind of stinks. And I know plenty of DM's who don't use feats, so my situation isn't exactly uncommon.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.