It'll be a total shitshow. The two warring factions will get coins on both chains, so they'll all start dumping on the chain they don't like while alts take over.
This. Bitcoin's dominance is already lower than it's ever been. With a hard fork, at least one alt is bound to surpass its mcap, which will be catastrophic.

For now, I don't think that they "take over". There is no real competitor in the top 10 of altcoins. D*sh is instamined and semi-centralized cr*p, ETH is not about payment but more a B2B solution. NEM is the Japanese Bankster Coin (JBC) and has a shady, blockchain-bloat-encouraging algorithm. Monero ... is OK but has even more scalability issues as BTC. Ripple and Byteball are totally centralized. LTC? Don't think so. So there is no real contender but all together could capture 50% of market share - and that would be already a hit for BTC.