No, it is increasing. The value in the UTXO that traces back to segwit transactions -- even if not currently on a segwit address -- is monotonically increasing. The prize available to the miners for reverting to the older rule set does nothing but increase over time.

I'll agree that it is by no means known that they will do so. But it is also by no means known that they will not. Funny that for all the lip service Core gives to security, and the rather dim view that many if not most Core devs have of miners, that they would introduce such a vulnerability.