Dungeonosophy
Legend
The game rules specify how various aspects of the world, e.g. magic, function. If what happens in a novel or an edition-change alters the effect of something, then there are natural questions that would arise among the people living in the game world as to why basic facts are now different. Hence the need to justify the changes in-game (or just have it be a retcon, which I find to be the more unpleasant option, though it can still be made to work).
Exactly. Mystara is my favorite published setting. It was switched from Classic D&D to AD&D2e without employing any in-story event. In one year, the Poor Wizard's Almanac used CD&D stats, and in the next year, it had 2e stats.
But, the CD&D cosmology was very different from the 2e cosmology. References to Mystara's place in the Great Wheel Cosmlogy started appearing...but CD&D Mystara definitely had a different cosmology...so what happened?
I remind the readers again that I'm not advocating that WotC employ more any more in-story Spellplague events. I'm not opposed to all those who are chiming in about how they hate in-story justifications of rules changes.
Hear me out:
3e recognized that each World has its own Cosmology. This was an advancement from the 2e policy where every setting was shoehorned into the single Great Wheel Cosmology. The fact that each 3e Cosmology could be reached through the Plane of Shadow was a "meta-cosmology", or a "trans-cosmology".
What I'm advocating is that WotC designers explicitly acknowledge that each rules iteration is its own Reality. This concept is a "meta- or trans-cosmology". This concept comes from an obscure Dragon magazine article written by the TSR-era Classic D&D brand manager. A link to the full article is included in the OP.
If this concept is dusted off and widely recognized, then the 5e designers won't feel pressure (whether it's a "need" or a "want") to justify the switch to 5e with any in-story events. Instead, the 4e Reality of Forgotten Realms will simply no longer be supported, but will still exist "off screen". And the 5e Reality of Forgotten Realms will simply come on stage. This is similar to the "elseworlds" of Star Wars or the recently re-booted Star Trek movie.
Like the other Realities listed in the OP, the Fifth Edition Reality existed all the way from the beginning of time. The ToT and Spellplague still happened in the 5e Reality of (Abeir)-Toril, but in the 5e Reality, Forgotten Realms always was viewed through the "lense" of the 5e rules.