Hi. I can be wrong. I don't follow these two closely... but I think that the proposal to Alfie, or not doing by him for 10 years, is about the total control that he clings to.
He knows Zoe won't propose. He knows she is desperate for him to do so. All their fans are eagerly waiting. All is set in his corner.
Alfie is less famous and successful than Zoe ( it is a throwaway comment, casual gossipper here). Even though in his mind he is entrepreneurs/gadget and fashion guru/Pro vlogger etc. He is and never will be Zoe Sugg. To too many he is Zoella's boyfriend! Now, that hurts (speculation).
Then, if he proposes...she gets the spotlight as the *bride*/wedding organiser, she'll as a bride to be, would get more deals with the brands than him (presumably). She is giddy and she gets what she wanted from him for her. Getting married, another milestone, attention and self esteem boost, smugness....superiority (?).
Now Alfie, after proposal, is left with pretty much almost no control/change (apart from best wishes comments, there will be "nothing else" to make it worth it for his ego). On the other hand, alltheir fans are asking Zoe about the ring, wedding,dress, food menu, music, flower arrangement, guest list.. Everyone watches HER IG impatiently. Zoella's channels GETs more views. Zoella gets interviews! And Alfie?
Alfie thinks that he won't have the fair share of the deal aka attention, control, brand deals, image, all those things that Zoe will benefit.
I can be wrong because I don't follow these two very closely. But to me it's very clearly about the last string of control/attention and superiority that Alfie has over her.
Additionally, he might rightly believe that Zoe waiting for the ring will make her more humble and *trying* as a girlfriend. Without the proposal or wedding, the current situation makes him think that he has the upper hand.
It's Alfie's ego that stops him in my judgement.