Renewing the certificates seems technically pointless, but some organizations/federations require it.

Rotating the keys would make some sense, but just swapping the cert for a new one issued against the same keys doesn't. It's the easiest way to fulfill those requirements, because you don't need to synchronize the metadata updates, the signatures are always valid with both the old and new cert.

All my x509s for SAML signing are self-signed, and all my self-signed certs live for 9999 days and I plan to let someone else figure out the fallout from that as I'm going to be /retired/. No one has ever really complained. Some IdPs I've integrated with use certificates that are signed by public CAs and it's always a hassle because the ergonomics around it are terrible.

IMO, I think rotation is wildly useless too. It might make sense in a world where my signing certificate was decoupled from the metadata someone else has to very likely load by hand.