Is it green, if a passenger takes up another passenger's space with a bike?
The alternative is a car journey, and it’s only zero sum if the train is at 100% capacity. For the person without a bike they have the option of a bus to remain green if train full. But unless advance or with bike you never book a specific train and therefore no one displaces you from a train.
I have yet to see a UK train at 100% capacity since the pandemic kicked off. Unfortunately the same cannot be said for the motorways.
The pandemic isn't normality, and besides, that's an effect of the public's view of getting infected. It can change and will do.
As for the 100%, the problem with bike spaces is that they aren't people spaces. So the train has less capacity, unless people stand in the bike areas. Which is fine if you run a rail system that isn't at capacity, but lately parts of the network have been or are being reengineered to create more capacity. Take out first class, take out tables, take out buffet cars, take out shops and replace with trollies, have airline seating arrangements, more carriages, more platforms (like Reading, which was a huge bottleneck on the GWR). If you have done all that to increase capacity, how strong must the justification be to take capacity out for bikes?
There isn't one alternative, there are several options - one of which its bike on train, another is a car journey, another is not to travel, and another, to have bikes at each end of the journey (possibly generic hire ones).