That's a false equivalence.
Indeed. Motorcycling is much, much more dangerous than cycling. That being the case, mandatory motorcycle helmets are proportionate, but mandatory bicycle helmets, other than
possibly for children, are not.
The reason it is more dangerous is the higher speeds involved.
Exactly, anyone who implies a helmet doesn't protect someone is clearly an idiot.
Nobody said it didn't. The point regarding compulsion is that it discourages cycling. More cycling is better for lots of reasons - most notably it means fewer cars (and it's cars that are dangerous), and also "critical mass" i.e. cyclists are safer when there are a lot of cyclists.
Probably most cyclists do wear them these days. But having to wear one reduces the ability to use things like the London cycle hire scheme spontaneously, as if they were required you'd need to carry it with you just in case, and they are more than a little cumbersome.
Most people who argue for compulsion fall into one of two camps - doctors, who tend towards "nanny stateism" fairly naturally because they see a lot of bad stuff, and in far greater numbers people who are anti-cycling and want to discourage it - it's usually accompanied by "and tax and insurance".