Extremely light weight? It may be complex, but x509 is based on ASN.1 and one of the most compact and efficient way to represent and handle certificates.

There are so many tools supporting it properly, and many of them are extremely hard to change (especially HSMs) because there are actual dependencies, that I wouldn't bother.

The problems are mostly in SSL & TLS protocols, not in the certificates. We should get a new alternative that would be designed to be easily implementable, and it should get proper reference implementation (with proofs of correctness, which by the way are available for X509... That's the only part that is verifiable afaik).