I think the arguement lies something along the lines of the following, overtime more and more coins would be held in segwit rather than in on chain addresses (as currently). This results in an incentive for a miner with sufficient hash power to fork Bitcoin again into a chain which no longer provides segwit functionality - they would then "spend" the segwit coin to themselves on their new chain - all those who had held their coin via segwit recorded transactions would lose their coin and there would be no recourse. It is assumed that this will happen as eventually the value of the coins held in segwit related accounts will be so big the economic incentive to do will be to big to ignore.
I fully admit my understanding here is incomplete/limited so I maybe misunderstanding or misrepresenting the argument.
That potential value doesn't exist if the fork is not followed. Like some here are repeating, cryptos are only worth what we want it to be worth. BCC might be worth something in the future, in part because it is done "the right way", BTC holders get BCC, no ifs and buts.
It's a hard fork but it's not screwing people.
The fork you're describing has as a goal to screw over millions of people. Why would those new coins be worth anything, who would put money in that shitcoin ?