It will require some courage for the first PM to agree to go on PM 1to1. It certainly carries a degree of risk. However, the programme would be such a clear step towards more democracy, that a PM who did decide to do it would gain significant credit.
A PM may need some persuasion and the petition on this website is intended to help persuade. If you like the idea, please sign!
There are clearly benefits for a PM which are likely to outweigh the risk. They would have the chance to speak directly to millions of viewers without their message being framed by the media.
If a member of the public on the programme is very hostile, the PM can reason with them. They are unlikely to persuade them, but they might persuade those watching.
Once one PM had done the programme, all future PMs are likely to feel they have to do it too, just like they have to do PMQs in the Commons every week.