I don't know about todays AIO
if you had the TDP you would see performance start to taper off
And it was a quick degradation once I noticed. Especially in the summer.
I don't understand what "
if you had the TDP" means. If you mean the system ran at or near maximum TDP for extended periods, then cooling efficiency dropping could have been due to many reasons - even if not running taxing programs. For starters, ambient (room) temps play a major role is system cooling, even with water cooling. Ambient temps naturally rise in the summer if the room is not environmentally controlled to maintain a constant year-round temperature. You have to get into "active", thermostatically controlled, alternative cooling solutions (such as refrigerated cooling options) before ambient temps don't matter.
Plus, age can easily affect the impellor rotation speeds which will affect cooling too. And TIM (thermal interface materials) efficiency drops a few degrees (typically ~5 to 6°) as they age. Other factors include, but are not limited to fan rotation speeds as the bearing age, and dust.
Point is, what you noticed is typical, regardless the age or even type of cooling. Even as a characteristics of electronics in general, efficiency tends to drop with age.
The biocide fouls in prolonged high temperatures.
Well, first, nobody is talking about "prolonged" high temperatures.
Second, the temps the OP is experiencing are not even "high".
Third, a primary advantage to AiO coolers is that they are sealed, closed loops straight from the factory. Fouling, or contamination occurs when oxygen gets into the system. Unless other physical damage occurs that creates a leak, that won't happen in a sealed, closed loop.
This should result in a pump failure, not a leak
Who said a leak was the more pressing issue?
Well, you did reply to a leak comment.
In any case, this should not be a problem anyway, in a sealed, closed loop! Not unless, somehow, the loop became unsealed. And that would be an exception to the rule, not the rule. And for the pump to fail from this, the toxic biohazards

growing inside that system would have degraded cooling performance long ago - to the point, surely, the user would have (or certainly should have) noticed unacceptable temps and done something about it.