I, too, wonder about degradation. Leaking is nice because it's a visual indicator for the customer, and unless they have specific complaints about ride quality I don’t usually mention shocks or struts unless I can see they're leaking.
But yeah, you can absolutely have no leaks but a significant drop in performance. Furthermore like looking at yourself in the mirror every day, it's gradual and you don't necessarily notice the aging as it progresses.
This is why I won't usually use the budget options unless it's a beater I think is likely to die for other reasons within ~2 years.
For nicer vehicles I try to stay with OEM, Bilstein, Mando or Sachs, although I'm still vexed by a recent Sachs failure in under 11 months where the struts just became super HARD/HARSH -- this was on an '02 Forester.
Also, old Subarus are disturbingly common here (like 2000 to 2010) and good strut options are surprisingly difficult to find. So usually I'm forced into cheap and hope the ol' Boxer blows up soon
I've tried stretching my legs from RA, but Amazon is 99.9% questionable junk like O'REDY and similar ilk, and Parts Geek often doesn't list struts a-la-carte but
only quick struts.