The problem is not xGPL, it's the copyright transfer. If you don't force copyright transfer on the patches you accept, you can't relicense a code overnight.

All "xGPL to shenanigans" incidents have underlying copyright transfer problems. Recently, an emulator had gone the same way. They asked for copyright transfer to be able to relicense from GPL, and some folks here have used derogatory adjectives for people who didn't want to transfer their copyrights.