He can get a fat cheque, with a big production team and set his own terms if he's in a position to do it.
Or - he can literally make the same stuff, but have it under BBC terms.
I feel there is something better than YouTube for these types of shows.
I think the jump between TT/YT and broadcast is too big.
Assuming they are making good money, I really don't see an upside to moving the show to broadcast television.
Though I completely agree with what you are saying as whole, being picked up for television could be a bad thing for content creator. If they can manage an audience and can work out ways to monetize not just on Ad revenue that can be a lot better in the long run.
When a network backs you, the advertising, promotion, reach is substantial.
Just because someone is on YouTube and theoretically 'anyone' can see him is not actually material reach.
When you're making a BBC/Sky show, that opens up doors to significantly more places, whereas today he's just 'some youtuber' which is nice, but it's not big.
And of course follow on deals and residuals.
For example: he gets booked on the Talk Show circuit, BBC interview circuit, his profile is raised. He maybe can get access to CERN, or the Moscow Ballet or whatever, things like that.
The various auto enthusiast shows from the UK are massive. Those hosts make a lot of money.