It's not scalable for you either. If everything becomes an error, you can't go a single day without fixing it and everything the compiler + your dependencies - aka, just some other people you've never met - complains about.

In particular if you update a library and they deprecate some functions, -Wdeprecated will start emitting some warnings, which are now errors. Do you think you can fix that in a day? It could take months to fix something that's literally not a problem yet.

By the way, I couldn't build MariaDB on this one machine because some plugin we don't use has -Werror set and fails with obscure C++ nonsense.