Your post looks as if sponsored by Denying the Antecedent logical fallacy.
http://www.fallacyfiles.org/denyante.html
Re planes. Planes were in those early games because of the memory limit. We don't have that memory limit.
That may be the case, but gameplay implications were much deeper than your oversimplification suggests. The two planes were parallel to each other, so if you traveled on the normal plane, your position relative to Myrror automatically changed. Later it could come into play with spells like Plane Shift, or even creatures which could switch at will.
A bigger map is just that. Try playing football (soccer) on an airfield or similarly huge open space without walls. The bigger the playing field, the more you have to run around. If you go too far you'll have a loong long time to go back. In Master of Magic, on the other hand, your position can be close to two differerent places. With its two planes, MOM had nice implicit shortcuts. Elemental with its huge maps won't have such shortcuts unless you explicitely add them. And because it was a fundamentally separate plane, you couldn't peak into it via high vision range and similar measures. Not without conquering a tower.
Finally, the idea of different planes is very fitting to a fantasy setting - or do you suggest they've come up with that to solve a (book's? author's) memory problem ?
