The vast majority of Standard class passengers will be unaware they have an extra coach.....
I would notice!
.... but FC▸ passengers will raise merry hell on the train and in the media if they are forced to stand.
Agreed if it happens
A few 2nd class peak passengers will get to sit down. Ok, a few more people might be able to fit on the trains at Reading.
More than a few. That would be the upside
But I doubt it'll compensate for the huge numbers downgrading their First Class seasons to standard (or switching to the car).
If all the displaced 1st class passengers moved to standard there would still be additional seats in standard class.
And at off peak times, 1st will still be even emptier with fewer upgrades and advances!
I don't see the logic in this. Advances will be made available to match demand. If indeed they do remove weekend upgrades then there will be correspondingly more advances.
FGW▸ make money by filling seats not running them empty.