Where the idea might come from would be that many (particularly large) websites would have implemented AMP reluctantly, and currently incur extra maintenance cost for AMP as part of a deliberative ongoing cost-benefit analysis. This removes one of the major obvious benefits in that analysis (AMP pages may still benefit but it's harder to qualify), so there may be a stronger technical argument within companies to drop support.
> It will continue to rank by "page experience signals", and presumably AMP pages will continue to be the standard to beat.
This is a tricky prospect. A concrete "we will treat your AMP page preferentially" is a lot easier for tech managers to grok than "AMP stands a good chance of scoring well on metrics we prioritise, so may be preferred, if you believe you can't achieve these metrics without AMP".
On the flip side, it's completely conceivable that Google could continue to treat AMP pages preferentially while claiming it's due to proxy metrics, since their algorithms are not public.