Why does that matter, though? Whether time travel is rare or common, the DM can make sure it's accessible to their players. And if those players go back and alter the fundamental nature of the multiverse, that doesn't affect anything outside of their home game. It's not like it'll require WotC to re-write all their setting books every time some PC goes back and steps on a primordial butterfly.Other option could be a new setting based in "shards" or fragments of the First World, a patchwork demiplane within the elemental limbo, adding a lot of thigs from "Councyl of Wyrms" and some pieces of Mystara Hollow World spin-off. This could add enough space and "hunt territory" for all dragon species and even future new kaiju monsters.
Time travel is canonilally possible in D&D but they are extremely rare because the cosmology could be too easily altered. If WotC wants to recover the chronomancers then they will need a lot of time and work to understand how the future metaplot could be affected.