A lot of software, and thus build automation, will break due to certain features that become warnings or outright errors in new versions of C++. It may or may not be a lot of work to change that, and it may or may not even be possible in some cases. We would all like there to be unlimited developer time, but in real life software needs a maintainer.

I'm not talking about software compiled by the compiler having a higher default.

Warnings becoming errors would be scoped to gcc itself only, and they can fix them as part of the upgrade.