I don’t think it’s a terrible suggestion, but I’m not sure
The only real thing would be that it needs to be proportionate, operators should only be allowed to charge for bicycle carriage if they actually make some sort of additional provision for them i.e. a specific bike storage area on the train otherwise what have you paid for the right to block walkways or have your bike fall over?
Also I think if you use a folder that can be stowed underfoot or on a luggage shelf then you should not be charged, as that passenger has no greater space claim than a foot passenger with a bulky rucksack/suitcase.
Nobody who takes a bike on a train really wants to be in the way or have it damaged when 3 others fall on it mid journey, I think cyclists would pay a small extra fee if they were actually getting something more for their money.
Of course the opposite argument is that bicycles should not be charged for as free travel with them encourages people to use the combination of train / bike reducing the use of cars, and all the good stuff that goes with that, obviously where additional provision is made then additional cost is incurred (say on a peak hours commuter service), Where it can be shown to be necessary perhaps operators should be able to agree a cost and apply for a subsidy? That’s gt the DM readers up in arms though…