A one-year maintenance cycle is in no way "trivial". Even if good design and construction stretched that out to the scale of decades, it would be totally unacceptable. Even if time-before-failure was stretched out to the scale of centuries, the consequences would be catastrophic when they started failing.
The old waystones have been around for millennia, and that's what they're going to be compared to. Even if you're willing to accept the embarrassment of designing something that compares so unfavorably to the ancient design and mandate a regular maintenance schedule, there are many ways such a schedule can get interrupted. Beast tides, Waaghs, famines, plagues, civil wars, corrupt nobles, Skaven shenanigans, etc. Records get lost, people forget why they were supposed to be maintained, towns get abandoned.
Assuming that this is the nadir of the control and organization the forces of Order can exert and from here on out things get better and continue to stay better and there's never a large-scale breakdown in civil order is a terrible bet. Not only that, but once such things happen, then the waystones also start failing which turns things from a mere disaster into a catastrophe.