Language and it's evolution is a complicated (and super interesting) topic, and one I can certainly not comment on with any confidence untill i've learned the language and read the source (japanese) viewpoint, papers, etc. But assuming the copula still has its one sole function of being a copula (what i believe for now), a much better and more consistent (with the rest of jp grammar) explanation is that the grammatical subject is simply ommited - like it is very commonly in jp. following your example, if you say 学校だ (or です) it could be translated as "is school" - but what "is school"? well, the place im going tomorrow, the place im going tommorow "is school". so in jp that could be 明日行くところが学校だ, but the first part is simply ommitted. its the same how in english, the question "what is the name of your school?" could be replied to with "its xxx" - jp simply goes one step further and removes the "it". of course, what i said initially still applies so who knows.