Regarding school closures you do not have to worry about a school like Rugby. On the whole, although there are exceptions, it seems to be very small boarding prep schools that suffer or other cheaper independents which are close to top schools.
As for removing the bursary I believe they have to have a "get out clause" and this is why it is stated in the paperwork. Realistically, unless DS academic performance diminishes dramatically or his behaviour and effort are poor then it is highly unlikely that they would take away any bursary. When offering a bursary they generally offer it to the strongest candidates with the assumption that they will always have to maintain this bursary for the full 5-7 years. It would be foolish of them to remove the bursary because more applicants needed help because you would then take DS out and the money and other resources they had spent on your son would be for nothing.
I do now how you feel though. I hate DC being on scholarships/bursaries because if their performance drops I always get stressed that this means they could take lose their awards. I'd much rather be told DC got a C in his latest report and shrug it off rather than panic whilst reading his not so good report card. And it is so easy to pick up on every negative comment because they are receiving a lot of funding.