I don't really think that anybody would leave or divert or shift (whatever you call it) their mining power to any shit coins unless BTC's security becomes questionable and encounters the possible attacks OP started talking about. It's not just the 51% attack alone that could destroy BTC but the interest of miners is one another major consensus that could leave you, me and everyone empty handed as well as scratching our heads off if they ever decide to drop this Bitcoin thing right there because of their personal (possible) losses that they may face because of lower prices and not being able to even break even, and as many miners mine for economic gains too, if they see more profits while mining other 2 coins you talked about (and even any other coin they think to be the next Bitcoin and what not), it will literally do a lot of harm to the economy of BTC and may slide it down from its (1^st) position that it has been holding since a decade.