And by that I don't mean nobody will play it or that it won't be fun to play. There will be tons of people who will play it. For quite some time I am sure. The problem is, it's at a stage of the game, that is stuck at a certain point. Imagine if the current game is stuck at where it is right now. No new content will come out. You won't be able to reach a higher level than level 120. No new gear will be added to the game. On top of that, you already know everything that is in the game. You've played it 15 years ago. The eventual interest will wear off quickly and it can't be resparked with new content. Even if they have said that they might do additional content for the classic version of the game, they'd be really stupid to do that. Every content they develop for Classic WoW could be content developed for the current game.
The game is effectively dead. It is like a branch, that has separated from the tree and is lying on the ground.