to build on diffbeer703, this is about identity and integrity. the identity verified by the certificate is trusted because the signing CA is trusted. if the CA signs certificates for 100, that generally is not trustworthy behavior and so the certificates they sign should not be trusted.
it is unlikely that a server identity will remain the same, let alone in existence for 100 years. also if the server is compromised its certs can be stolen and used to identify another server as yours. short lived certs make more frequent cert verification necessary thus making
server identity theft less of a threat.
if you manage the entire chain then, it is less of a concern especially if the only person needing the server identity verified is you. the more people depending on the server identity the more important it is that good practices are followed by the CA.