[Filthy_casual]

This brings up another point in favor of "why not".

If existing players who are willing to buy new expansions whenever they are available migrate to vanilla servers, they might as well opt out of buying the expansions, since adding new content in the vanilla servers will make them non-vanilla.

Huh.

[Margh]

A prudent developer might make the expansions a mandatory purchase that adds the new content in cosmetic ways.