Miners absolutely care about market value (honestly moreso than they care about fees -- the bulk of what they make is still subsidy). Scare off market value with frivolous forks like this and miners may not make any less fees but still get less market value.
Bitcoin being hard to change and unlikely to do so for frivolous reasons isn't just a nice to have. Get rid of it and the whole value proposition falls into question. Are my transactions permanent? Is 21 million a cap? Is a presigned transaction I make today ever going to be able to be broadcast?
There's always some doubt, but when we see first hand that the whole network gets yanked around by an angry hoard of retards, it goes from being a nagging worry at the back of your mind to a real and present danger. No counterparty risk turns into not knowing what small group becomes the risky counterparty you now have to worry about. Bitcoin never did away with trust -- Satoshi solved the matter of trust by distributing the trust across a big enough network of diverse people that it was unlikely they'd ever be able to go along together to be a risk. If that premise falls apart it all unravels.
