I guess we are talking about the possible conflict between coherence with the previous lore and the necessary changes to update the gameplay, for example to add sorcerer and warlocks in Dragonlance.
Oficially the multiverse or parallel worlds are canon in D&D. WotC needs the lore to be enough flexible to allow ideas by the players themself, even the craziest ones, for example a reason to explain an isekai in Krynn. Let's imagine a group of ordinary citizens from Kamigawa: Neon Dinasty losing their lifes during Phyrexian invasion, and their souls are "reincarnated" into tinker gnomes, who suffered a fatal accident in one of their laboratories for a fool experiment about planar gates. These tinker gnomes remember their past lifes in Kamigawa because there were "variants" of the tinker gnomes who almost don't survive the accident in the lab.
Some times the lore has to be sacrificed to allow adding new elements of crunch. For example I could invent a Krynn where psionic mystic are possible. Or a variant of Dark Sun with incarnum soulmelders, martial adepts (crusader, swordsage and warblad), and vestige binders, and the PC species from 3.5 Expanded Psionic Handbook.
My theory is this Vecna event could cause some radical change in the D&D multiverse to can explain later possible incoherences or retcons, for example visitors from the Gamma World RPG, or other franchises (Why not a Evil Dead-Ravenloft crossover?, or Ravenloft with the manga "I am a hero").