Let's face it , Windows Phone needs all the PR and reputation it could get , and with the kernel being even more optimized and compressed ( wp8.1 update1 and w8 update 1 ) Microsoft is working on a better support for the low end and o, naturally , older hardware , and unless the 530 dethrones the 520 or WP 9 is so good that people ( who were screwed over btw ) would buy a new phone or they decide to go all iOS on us , I think there's a 80 % chance the 820+ will get WP 9 and 60% the 720- will get it too but again if they go apple like , it might lack some features especially key features because they need to sell new phones while keeping the old one compatible.
The best argument I'd support my opinion with is universal apps , they want to give access to a maximum of users and they realize that some people only use / can only afford second hand phones or older models .