The latest ManageSieve draft already does that ;-).
Sorry, forgot that was in there.
Given that support for SRV records is already required, old servers that
are listening on port 2000 can be made to work with a new client that
defaults to a different port simply by adding an appropriate SRV record.
And an old client that goes straight to port 2000 on the named server
isn't going to interoperate with a new SRV-dependent setup anyway. So I
see no compelling reason to continue to mess around with port 2000. Let's
just get a new port assigned, put it in there and be done with it.