I've done the bike-on-train thing many times and in many countries. The issue isn't just the space the bike needs on the train itself, it's the space the person needs to be able to get a bike on board without blocking the path and the infrastructurerequired to get the bike right next to the train. Trains fit for many bikes need wider doors, more doors (that costs seats), alignment between platform and train becomes even more important, that the platforms are very accessible too (there is often, if you're lucky, 1 elevator to the platform that fits 1 or 2 bikes at a time, that elevator gets jammed up and competes with wheelchairs and childstrollers and large suitcases very quickly) et cetera. Many smaller stations still have 0 elevators of ramps, only stairs. The only somewhat convenient bike on a train is the foldable bike, but even that creates the hassle described, tho less. I try to avoid taking my own bike on a train (and I think taking your own is usually too cheap compared to a person-ticket and the hassle taking the bike creates).

Anyhow, I think 1 person + 1 bike = 1,75 seats is underestimating it a lot.