It's actually a lot closer to the 2e map. For one thing, the 3e map includes trails as well as roads, whereas the 2e and 5e maps only show roads. For an example, compare the roads in the region between Baldur's Gate, Elturgard, and Dragonspear Castle.
I didn't look at my 2E maps for my original post, and I should have, because I agree with you.
Almost all of the changes made during 4e -- and even some of those made during 2e's Time of Troubles (eg. Bhaal is back!) -- have been changed back by the Sundering. The only ones the SCAG mentions as still being around are the floating city of Airspur and the remnants of Tymanther. Oh, and Elturgard. ... The Sundering event was designed to restore the Realms to more or less pre-4e times (and, in some cases, even pre-2e). Hence why the map looks the way it does.
Respectfully, yes and no. The Sundering is the separation of Abeir and Toril because of the restoration of The Weave. The recombining of Abeir & Toril was an accidental side effect of The Spellplague when the Weave failed. Thus anything that is Abeir-on-Toril goes back to Abeir, and we have to assume Toril-on-Abeir comes back to Toril (hello Evermeet?). Except for a few locations without explanation, as you noted (designers' privilege to keep genasi and dragonborn in 5E?). Clearly with the Weave restored, the plaguelands go away. [Sux to be a follower of the Order of Blue Flame I guess!]
Thus changes wrought by Abeir being present are removed by The Sundering, but not the mundane effects that happened around that (imho). The giant sinkholes were caused by earthquakes collapsing the Underdark, not be pieces of Abeir crushing the Underdark caverns like giant meteor impacts. Chult became an island because the oceans were tumultuous and swept away most of Samarach, Thindol, Tashalar and the Lapal region jungles. Luiren and Var sank into The Great Sea by the same events. Halrua was blasted by all the magicks going awry. The Shaar became a desert because of climactic changes. I doubt that these changes will be undone - but I could be surprised!
So the giant mountain of Xian southwest of Nathlan - yeah, that's gone. Earthmotes? Yup, gone as well. But the devastated region of Halrua? Sunken Luiren? Island of Chult? I don't think those have changed. If the SCAG map is truly officially 5E, then how the devil(s) and angel(s) did all the changes occur?
Yes, there were changes from the 1E to 2E maps, and fairly large changes from 2E to 3E/3.5E maps. The 2E to 3E/3.5E changes were explained in meta-game terms: reduce areas no one plays in and increase areas where people play in. Some folks didn't like that at all, but at least we understood why the changes occurred. We have no understanding of why the Farsea Swamp suddenly split back into 2 different marsh regions, or the giant gaping sinkholes vanished. Or glaciers suddenly reappeared and lakes vanished. Such major geographic changes need to be explained, at least in metagame terms.
I guess what I really, really want [please, no Spice Girls comments!] is for Mike Mearls and Jeremy Crawford to come out with an "official canon changes from 4E to 5E Forgotten Realms" guide and clear up all this mess.
