People, I think, are more forgiving of changes like these because they made FR more unique, not less unique, which I understand. That doesn't mean they weren't huge changes.
I disagree. Monkeying with the planes strikes me as a way of stamping authorial identity on something that already exists, whilst changing nothing important. The effect of the planar change on the look and feel of 3E FR was pretty much zero unless you explicitly took the PCs planehopping.
4E FR on the other hand hamhandedly shoehorns and retcons in entire new aesthetically wonky 4E races like eladrin and dragonborn, kills off all the NPCs, nukes certain areas whilst leaving others unrealistically completely unchanged, and borrows rather obviously from Eberron in multiple ways, as if that wasn't doing as well as expected so FR can be surrogate Eberron. And the map changed a lot and looked bad to boot. And FR became a place that former non-fans of the setting now liked, at the expense of existing fans who walked away in droves. And the 3E FR book is one of the most beautiful and well written setting books ever produced for any RPG, which throws it's successor into sharp relief in comparison.
Oh, and the setting got simplified so that authors wouldn't have to do their homework in writing for it, too. Nevermind that rich detail of Realmslore was a main draw for FR fans. And they killed Mystra again, which was just cross-eyed stupid unnecessary to those who had been following the setting, and looks like a thought bubble from someone unfamiliar with FR who just tuned in.
This was no From the Ashes reboot of Greyhawk, which has many redeeming features from the fan's perspective (even as it did turn many off). Most of FR 4E's virtues are only in a meta sense, like "shoehorns 4E races and classes into FR" and "makes FR less detailed so as not to intimidate authors and newbies". In other words, compromises made to satisfy political needs that have little to do with producing a quality setting. Yes, this has been done before with all assassins and monks disappearing, but what 4E FR did is much more on the nose and in your face than that.