@Angelicart but why is it a bad thing if an Old Etonian won't go for a random application? The idea of open applications is not to give an advantage to certain groups, it's to let people opt out of having to make a college choice. It also slightly expedites the reallocation process which happens anyway before interview in most subjects nowadays, which means that by the time people are interviewed, most colleges have the same ratio of applicants to place.
(I agree, by the way, that an Etonian won't do an open application on the whole. This isn't because they are clued up, but because top private schools are institutionally invested in selling the idea that they know a lot about Oxbridge. It's rather like private companies who charge you money to offer advice on which college you should apply to, which I'm equally dubious about. I'm not disputing that people from Eton get a lot of help and support with applications compared to people from a normal state school, but college choice is not an area where that help and support makes any difference.)
In practice all colleges are 'popular' and none are 'unpopular' in the sense that all are over subscribed, so it's just a question of how over-subscribed. There isn't a back door in by choosing the right college, just as you aren't making your chances worse by happening to choose the most over-subscribed one, because at some point in the process the system will even things out, whether through open applications, reallocation, or second round interviews. Old Etonians certainly do end up at a range of colleges, whether or not they originally applied to them.
Also the less over-subscribed colleges are in no way lesser options or rubbish colleges. They are usually just slightly further out of the city centre, so don't get as much footfall on open days. When people actually arrive as students, the location doesn't really matter as Oxford centre is tiny and everywhere is bikeable in 5-10 minutes, and there are advantages to being somewhere with a bigger main site and not having tourists try to barge into your room all the time!
Finally, it isn't straightforward for Oxford and Cambridge to 'copy Durham'. The legal set up of the colleges is totally different. Whether or not one likes it (and it's entirely fair not to), the colleges are self-governing and legally independent institutions and it is they who have the right to admit undergraduate students, not the university. At Durham the colleges don't have any independent legal status.