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 arguement.