User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-43631410-20200526210758/@comment-24006128-20200526211108

Because one of the points of the update is to replace two codebases, one which is modern but not as scalable and one which is scalable but not modern, with a single modern and scalable codebase.

Making it optional would defeat that point entirely.