The penalty is not "considerable", but it is not completely trivial either. But it's more on the trivial side. Part of the problem is that it is hard to measure, as it is very workload dependent. As to why it is not the default, Postgres is very careful and conservative by default, and not everybody was convinced that enabling checksums is worth the tradeoff, especially (IIRC) with the lack of real-world examples of people discovering issues thanks to these checksums. So yes, please enable and share with us if the checksums catch anything.
I think the argument is worth re-opening again, because (as far as I know), all of the companies providing Postgres support, whether completely cloud-managed, setting up a complex HA cluster, or just providing tuning advice, have enabled checksums by default for many, many years. So a big +1 from me to finally making it the default. It's a de-facto default anyway at this point. Cheers, Greg
