I'm not saying pre-fork and post-fork bitcoin are equivalent, or the same protocol version. That's a strawman. the term "Bitcoin" is obviously different from "ECMAScript 5/6" and "HTTP 1.1/2.0" in that it's not a version of a protocol/standard. Bitcoin is a term for the overarching thing, just like "Javascript" and "HTTP" are overarching terms for those, encompassing multiple standards etc.
Again, "altcoin" already has a term unrelated to protocol. "altcoin" means a cryptocurrency that is not closely related to bitcoin.
You're talking strictly about protocols and versions there (which, btw, bitcoin should just version its protocols), but that's not what "Bitcoin" means to the common man.
You're making my argument for me by comparing "HTTP 2.0 isn't HTTP 1.1", but they're both HTTP, not gopher or ftp.
Or, rather, ECMAScript 6 is not ECMAScript 5 because you can write code in the former that doesn't work in the latter, and probably vice versa on edge cases. They are separate languages. If you continue writing in ECMAScript 5, you're missing out on all the nice ECMAScript 6 libraries.
But with Bitcoin it's even worse - if you continue using pre-fork Bitcoin, you cannot reach consensus and thus transact with people using post-fork Bitcoin. They're not precisely the same protocol, by definition - they bare the same name but they're not the same protocol, they are different protocols and for the fork to win, the first protocol dies. The name is irrelevant to this.
Similarly, HTTP 2.0 isn't the same as HTTP 1.1, and claiming one is equivalent to the other is nonsense, even if they bare the same name.
I'd also argue that if the amount of hash power put into the network can be derived from the value of the network (trivially provable), the result of the economic majority choosing to adhere to different rules is that either the miners will follow or new miners will pop up quickly as there's a demand for them.