Provided you don't have 21 Monarchs who refuse to ascend, if a system was created that could safely manage up to 20 Monarch's worth of hunger madra for energy, what would be problem long term?
Long term the problem is that the founding assumption of <=20 Monarchs will be violated, if Monarchs are possible at all.
Either by the obvious Monarch creep as people choose to become Monarchs knowing there's already 20.
Or by unknown Monarchs screwing things up: see Northstrider pretending to be dead and thus being uncounted. Or both.
Any system that does not enforce the actual prevention of failure states will, sooner or later, succumb to them.
Now, if something can be put in place that flat out prevents Sages or Heralds from becoming Monarchs, or prevents anyone from becoming either Sages (probably not possible, since Icons seem to be from outside Cradle)) or Heralds (perhaps by changing world-rules for Remnants to make it impossible to fuse with them) then forcibly Ascending all existing violators (perhaps manually) and thus prevent anyone from being both at once, since only one is left.
Beyond that, yeah, auto-kick or make Cradle able to safely handle any number of Monarchs.