I've written before about setting caps on defenses, skills, etc. in GURPS.
A cap of Stat+10 as a limit dates back to 3rd edition. But why 10? Why not more, like 12 or 15, or less, like 4 or 6 like Talents?
On my drive home from a skill-training event*, I was thinking about the way GURPS has centered around +10:
-The TDM chart ranges from -10 to +10.
- the highest non-range penalties tend to cap at -10 (eyeslits of helms, chinks of armor over vitals, cannot see)
- stats tend to cap at +10 over base (humans range up to 20 in ST, DX, IQ, HT in most genres)
- Trying to do a long task "instantly" is -10.
There are few examples of very high penalties that aren't range, stacked penalties, or some combination of both.
So having a skill cap at Stat+10 is well in line with that. It has a symmetry - a true prodigy (DX or IQ 20) or a true master (Skill 20) can absorb the worst penalty and still have a 50/50 shot of success. A prodigy with mastery (DX or IQ 20 and stat+10 skill) still has mastery even faced with that penalty. That's not unreasonable as the true maximum.
I think that means Stat+10 makes sense as a potential cap.
* Aka, martial arts class.