A large chunk of Frostmournes player base will also not be bothered to play this new realm just because the prev expansions are very lacking on many levels. be it challenging content, class customization, class balance, PVP etc. The whole idea behind Frostmourne is to have challenging content and try to progress every tier to the best of your ability. I don't see this happening for Vanilla at least. Tbc might be different but that's a long way in the future.
Another thing about this decision is that you're not giving an alternative for Frostmourne players to play on. You say there will be a beta for this new prog realm in the summer, which means the actual realm won't launch before the fall (at the earliest?). You can expect an even larger part of the community to stop playing, find new servers etc. because of this as well. Personally, I am not willing to wait for something like this, along with many other people I've gotten to know over a few years here.
I think you should seriously reconsider this decision. My suggestion, among many others, is to just copy/paste season 3 into season 4, this will take some of your resource pool, but it is definitely a good alternative. This way you can maintain your player base and at the same time keep your community happy.