I don't think that we can ever really expect any single standard to fulfill all of the expectations from users for the features that they want. Vendors that want to distinguish themselves from open-source platforms are therefore going to have to step outside the scope of the standards in order to meet their customer's needs. SIP was originally meant to be simple, but is now trailing a long list of RFC's and even more semi-proprietary iterations. Simplicity is getting thrown out the window in favor of demand for features.

The best we can hope for is a reasonable general agreement on baseline standards and just accept that 100% interoperability of extended features probably isn't going to happen. We may be able to SIP trunk multi-vendor systems together but full cross-platform feature functionality is likely to be limited.