You could build a custom loop and run it with norprene tubing (just like sealed ones which dramatically decreases evaporation), and use corrosion inhibitors + biocide just as they do, and never clean it just as they dont. Or even with normal tubing and 150ml reservoir, it would take 5+ years before needing to add anything.
And both your custom loop or the sealed all in one units will slowly get oxidized blocks, but the effect on the performance is less than 1C over couple years. But eventually some air will accumulate in rad, especially in factory sealed without reservoir but again performance wont take that much of a hit.
It isnt that the factory sealed ones are going to last longer without cleaning if you build yours the same way, it is just the factory sealed ones accept the eventual small loss in performance and the internal build up of crud vs people that build theirs will simply clean it because they have higher standards and they can, especially when upgrading their equipment and the loop is broken down anyways.
EDIT:
Water vapor transmission rates
Tygon 3603 wvtr = 12.4 g*mil/100sq in/24hrs standard testing
tygon b44-3 wvtr = 5.8 g*mil/100sq in/24hrs standard testing
norprene 60 tubing = 1.1 g*mil/100sq in/24hrs standard testing
So norprene is 6x less permeable to water vapor (6x less evaporation) than tygon plasticizer free beverage tubing 44-3 and 12x less permeable than tygon 3603. Also 1/8 inch wall tubing would half evaporation of 1/16 inch wall.