I'm sure lots of them wouldn't renew, but the ones that do probably renew for several months, which will make up for quite a lot. To make the figure I'd claimed you'd only need 20% to stick and sub for 5 or 6 months.
It may well not be double but even 5% sticking around for 6 months would be 30% extra income.
This isn't aimed at you but I'm finding the third and fourth pages even more embarrassing than page 1...so many people worried about whether he was complaining or not when the point was he's been paying himself whilst making money for charity...and also asking for money towards charity when it's a really inefficient way to raise it.
If most people spent more than 10 seconds thinking about it they'd realise that. hey ho.
Despite the title of the op, this subathon was not aimed as a charity fundraiser and has never really been promoted as such. It was something he announced after it had already started, several hours in (maybe even a day or 2 in, I can't remember).
It's just a normal subathon that took off and he is also donation part of the proceeds to charity on his own volition.