I agree with @Willeke that approaching this as a community move is wrong (excluding the stealing part, which is just not the case), even though I'd personally definitely jump ship (well more like board another ship) if there was a functional alternative. Even if you disagree, I think it's not practical to "transition the community" - it's not an entity but a ton of individuals. You need to get them over and you do that by getting a working and thriving alternative.

To that end it needs motivated people taking the initiative to set something up, then an announcement and if things go well, users and traffic will follow.