While the shop and membership idea is modest.
It needs to be refined.
Like I mentioned in another post as the servers are now there is no compensation for people that have payed 5 dollars for 30 days membership boosts and as it is there is no reason to ever pay 5 dollars again. Or rather, thats probably what some or most people are thinking.
Regardless, I believe there are two ways to go about fixing this.
Whereas it would be understandably hard to do this; Compensate players by adding time back to their membership for the time that the server is down. If this isn't doable there are certainly other ways to compensate. The big thing here is literally compensating what people payed for.
The less potent way to go about fixing this or rather, once again, compensating is to give members login priority or their own server (once again probably understandably difficult and out of the question.
The main point of this post isn't to force ideas down your throats as developers as everything you come across is definitely a challenge. Its more so just to make you aware that there needs to be something done for the paying player-base with memberships, especially if servers go down frequently.