I think one of the problems the BBC and its relationship with the TV Licence faces is the shift away from TV as it is broadcast to on-demand using TV equipment, for two reasons:
1. With fewer people watching TV as it is being broadcast, people will require licences less, and whilst licence revenue increased slightly last year, I think it will peak as the younger generations realise they don't actually need a licence.
2. The medium through which people gain information and enjoy entertainment even on-demand is changing. When BBC News 24 started, it was one of the main ways of getting live news as updates come in regularly and it provides it in a format other than text, which is previously the only hypermedia used on the BBC News website for breaking news. Now when you see a shooting happen in Woolwich, there are live streams of commentary (text, audio, video) on the website, which you don't need a TV Licence for.
Radio you can't practically detect users; it would be a shame to see the website develop a paywall, you can't charge for following on Twitter or liking on Facebook, and the web is so content-rich that the need to watch things on TV as they are broadcast is reduced massively.
I guess I have two points to make. The first is around whether we should encourage more TV-as-it's-broadcast viewing, and if so how. I don't feel we should specifically try and change viewing behaviours purely for the purpose of licence fee revenue; I think that's the wrong way round - the revenue will come if you do what users want/need. There are however some innovate things I'd like to see the BBC experiment with more, such as interactive TV where you get to crowd-control what happens, and 'play along at home' activities like the app for Million Pound Drop.
The second is, finally, around who should pay. Given this shift away from TV, and the difficulty in preventing and tracking activity across a wide range of online, digital and radio services, I don't think the current model is long-term sustainable unless the BBC finds a way to retain live TV viewing figures. I also think the current model isn't progressive. If you're unemployed and looking for work, the BBC's documentaries, news, weather, current affairs, etc are useful, and I also don't think poor people should have their light entertainment taken away from them because they can't afford a TV Licence (it would be around 5% of many people's weekly JSA). It's hardly living the life of luxury having a TV, as the Daily Mail would like to portray. I also think that whilst not everybody purchases a TV Licence, almost everybody consumes BBC media in some form each year. Given that pretty much everybody does, and that it should be seen as a public service (after some reforms), I feel it should be paid for through general taxation.