I'm actually pretty impressed mine were holding together at almost 25yo. Maybe they're actually awesome if you think about it that way.I haven't seen anything about them that shows that Toyota improved on the design. Some folks are putting in brass units. I don't know how they handle reacting to anything in the coolant but can't imagine they are having issues.
The PO of my vehicle had a shop replace them at 100k. I replaced those at 175k (9 years later) and they were still in very good shape (slightly discolored). At 175k I replaced all of the hoses around the T's as well. It looks like these heater T's are just something that should have been added by Toyota to a 90-100k Toyota service interval; but the Toyo engineers likely didn't foresee having this issue.
I pull the radiator and fan shroud when doing a timing belt because it gives me more room and I don't want to damage a radiator over 15 minutes of work and some cooling fluid. I plan to do the heater T's every timing belt simply because it's easy to go the extra mile and do my coolant flush at the same time.
You could also do heater T's every time you do spark plugs.
IMO the plastic holds up great for 10 years, but 20 years is just way beyond what it was designed to do. If Toyota had fiber reinforced the plastic or just gone with metal T's this would have solved the issue altogether.