Keeping different tags for them makes a distinction without a clear difference; it is too easy to assume that TCPIP means the protocol TCP riding on IP (the only way it is used now). In ages past, there was a proposal to encapsulate TCP over CLNP, but has been dead for years.
As YOU pointed out, it is possible to consider TCP/IP as a competing model to the OSI model (this was the way it was used in the early 90s); however, all other competing protocols (CLNP, Decnet, Netware, Appletalk) have died off, so essentially there is no other competing protocol in use. Furthermore, even TCP/IP professionals routinely use OSI Model and TCP/IP interchangably when referring to a layered protocol stack.
For clarity's sake, it is more pedantically appropriate to retain tcp and rename osi to osi-model if you want a tag for a protocol stack. We could do the same with tcpip, rename it osi-model, but that would mean retagging many questions which currently use both tags (when referring to questions about TCP). If we do this, we now have a tcpip that is of little use on StackOverflow (like osi is today).
My original proposal to delete tcpip went for 8 hours with two upvotes and three downvotes. I am content to leave the status quo, if that is the community decision.
It occurs to me after so many edits, that the protracted back and forth on this question is perhaps related to the bigger problem... to date, people are unwilling to put a clear policy in writing about guidelines for networking questions on StackOverflow (ref my +100 bounty that went untouched).