I'll be honest here - it seems like you want to have your cake and eat it too. That is, have a hugely detailed world with abundant lore, but also have a setting that's light enough for casual DMs to just feel like jumping in.
I haven't felt like FR has been that kind of world since the grey box - my favorite version, fwiw.
Actually it is my feeling that the realms were easy enough for a casual DM to just jump right in. Where is it that a new DM wishes to focus? Set an adventure in Waterdeep you might need some background. You want a port city setting, yet you don't want lore? Ok use Zassespur. If you THOUGHT however that the realms was too complex why then use it? There is nothing about any campaign world I find too complex. It may seem daunting at first until you explore. Start small, then work your way out, like you are making your own campaign world using a small town.
If I had my choice, I would choose the rich lore over simplicity in any campaign setting I choose.
If they want to play the old realms, there's a ton of material available for it. Literally, staggering amounts - which is either a perk or a problem here, depending on your perspective.
I agree with you that the Realms material will always be available. For some people I know, the realms is like their comic books, they want to see what is in store for the setting. They were not however expecting the setting to completely change.
For the 1e/2e conversion, FR underwent a big ol' cataclysm. It shook up the setting. I wasn't paying enough attention to know if there was a similar shake-up for 2e/3e. For 3e/4e... Well, lets face facts - the games have some different assumptions.
The changes to the 2nd edition realms were the gods, and some nations were turned on their head. IN THIS change, you have a complete change in morphology. Waterdeep known for its great harbour, suddenly has sunk its entire navy, and its Harbour is jammed with ships that now becomes a new ward. It is no longer your great merchant city. I can't remember all the changes quite frankly because I haven't opened the book in months.
If you want to keep game mechanics (particularly involving magic) consistent with world-logic, in a world which shows a historical narrative awareness of D&D rules changes, you simply have to change some things. I'd say the Spellplague and Returned Abeir are a pretty good example of changing things. So, with that in mind, let me switch it around - how would you have handled the 3e/4e Realms conversion?
Simply you have Eberron with many many fantastical elements. I would have left the landscape very much the same as is, but perhaps changed some nations around if I wanted to go a cataclysm route.
Most likely since the magic system changed, I perhaps would of made Mystra the guardian of the weave trying to keep it from fracturing. Somehow instead of the weave it rips into some realmsian form of powersources. Wars may be fought but there would not suddenly be utterly alien phenomenon.
Forgotten Realms was a good place to adventure where you could have your standard medieval city or fantastical magic city. Now the designers have decreed that the non fantastical is no longer fun, so they needed to make everything alien. For some reason WOTC seems to think they are really capable of defining fun.
4e was a majour change (most of which I am unapologetically against), but the Realms did not need need a change in form to conform to the new rules set.
So are you arguing that there shouldn't be a 4e Forgotten Realms?
That would be ... kinda shocking, frankly.
Well WOTC still may need to sell some novels...
4e forgotten realms is related to previous edition forgotten realms in only the vaguest sense. It is a different world using names from a previous campaign world.
For all purposes though, if I had my choice of having this remnant be the realms or WOTC not printing it, I would choose they retire the realms.