User blog comment:MisterWoodhouse/Pausing UCP migrations to resolve some critical blockers/@comment-45171933-20200904204724/@comment-24155515-20200904213835

The reason it's not optional is the version of MediaWiki legacy runs on is 5-10 years out of date.

It's like running Vista or even running Windows 8.0 instead of Windows 10.

You can't make it optional if you want secure, stable software that still gets tech support.