It's a very good point, and I'll admit that I haven't considered it.
The hand-waviest response is that their docs indicate that a percentage of fees will be burned, so even though validators won't collect a lot in fees, it will be deflationary. It seems to me like fees will likely need to increase OR another incentive will be introduced.
"We intend to burn a percentage of transaction fee in every block. This makes the tokens deflationary in nature and provide it a constant support in terms of its value at the protocol level."
I also wonder if the other Polygon solutions will have tokenomics implications (rollups, private networks).