I would expect that rather than convert one more first class to standard, you could just offer a service similar to that already tried and tested with Eurostar.
On the train, it's very close to first class - but you don't get all the same luxuries, like lounge access. It's also priced more like an advance ticket I believe, so no open and fully flexible options. That's perhaps one of the big differences that would stop everyone in first class just downgrading.
I can see how it work would, and not just on that route. I am surprised FGW hasn't considered trying the same, or indeed EC - or any Intercity operator.
Prices are always going to move upwards so offering more for your money (as against trying to cut prices) is the way things are going in most industries. You just need to justify price rises, and I'm sure that what you offer (as is the case in first class) is always going to be less than what you actually charge for it.
Just as first class users on flexible tickets subsidise others, I am sure the middle class service could also help keep standard fares lower than they otherwise might me. A win for everyone, as standard is still a pretty comfortable way to travel in most cases. On a new train, standard can be far superior to, say, first class on an older 1980s/1990s commuter train.