Currently the discord poll here: discord.gg/n3tpPn623J shows 92.57% of voters in favour of a new S4 which is a copy paste of S3.
Launch S4. Cut the timeline if you have to. But launch it and allow 92% of your player base to be happy.
Don't kill what they want, so you can take it away from them and replace it with nothing. Come May 1st, there is no Frostmourne and there is also no "vanilla progressive realm" either.
What happens when your development for the Vanilla server falls behind schedule, pushed back to Q3? Q4? All that time you are hemorrhaging new users and current user who prefer fresh and new player friendly servers. If Vanilla drops on September 1st 2023, that at least gives 4 months of S4 content with 0 overlap. Call it 6 months if you take a little longer to work on the new server. Why not run a double speed S4? Or just let them run in parallel?