Nothing stopping them from introducing TfL only fares at tube rates
Define 'tube rates' when you can't put the stations (other than Iver and Langley) in numbered zones.
Should also note that fares on the ex-GA lines that TfL Rail/LO run are on a different fare scale (shared with GA) to the standard TfL fares.
There's also, from beyond-zone 6, residual London terminals fares to Paddington that are cheaper than Z1 fares - should they go because they aren't 'tube rates'?
Also those mentioning Amersham to London are forgetting the fact that many Amersham to London commuters will in fact be taking Chiltern
Or perhaps you are forgetting that, as someone 100s of miles away in Yorkshire, you are just guessing what Amersham is like.
And perhaps those mentioning Amersham (like me) are doing so because they are very familiar with it and not just because it's part of TfL's empire a similar distance from Central London as Maidenhead and so a very apt equivalent to discuss when talking about TfL serving Maidenhead.
Overall, at Amersham, the Met is slightly more popular: 2.10 million (2018 tube stats) vs 1.992 million (18-19 NR stats), and those NR figures include travel northwards as well. Off-peak, Chiltern is very much the more popular option (since December 2012, when Chiltern trains overtook all-stop Met trains near Harrow meaning that it was pointless taking the Met even to/from Harrow, though they fixed that, but people got in their habits). Peak time - ie, commuters - the Met is more popular.
OK, the 56 minutes Amersham-KXSP was off-peak (so not fast), but that was explicitly said. Of course, with fast trains, skipping those stops means it's 2 stops further at Barbican where you hit the 55 minute same journey time as Maidenhead-Canary Wharf.
which is a higher quality service than the Met.
No it's not! Certainly it is 5-10 minutes faster to Marylebone (39-44 minutes) than to Baker Street vs a stopping Met (49 minutes), but that's about it.
The equivalent on the Great Western would be Maidenhead's residual GWR service, if it was run by 165s still (and 2-car off-peak) vs through Elizabeth Line service.