The other point that gets ignored in a lot of discussions about moving BBC Three to online is that to watch online you need an internet connection of some sort. These are very seldom zero cost - so you are effectively moving from a free-to-view platform to one that requires payment in addition to the licence fee to watch.
Well, yes and no.
The same argument regarding "payment in addition to the licence fee" could have been made in 1950 about the cost of renting a TV set, paying for a connection to the electricity grid, and paying for the electricity itself.
Just because we tend to buy TVs nowadays and don't tend to view an electricity connection as an optional luxury doesn't mean life was always that way (nor that the marginal costs have disappeared). More homes have broadband today than were connected to electricity in 1950.