I hate to break this to you, but that wouldn't happen. Paying a subscription does not guarentee timely updates. In my few ventures into MMOs, I've seen many glaring balance problems and bugs that took months to fix. Stuff that should have been easy, like broken quests and abilities not working right, or at all. I was paying $15 a month for this patching service, and did not see fixes to many well known issues before I left the game. I've had friends say things like, "I'd pay $5 a month for this RTS if they released patches constantly." The devs can only release patches so fast. And that's not always a bad thing, as sometimes rushing out barely tested fixes and balance changes break things along the way.
Just because nobody's done it right yet doesn't mean nobody ever would. Anywyay MMO's are a different animal - I think a lot of people that hate WoW stay subscribed because changing games means facing the daunting task of starting over at level 1 with no alts or guild or expertise. You don't really have that dynamic in other games. And even if you fix or improve the game eventually, by that time everyone else is gone (see:Vanguard) and MMO's with no people are no fun regardless of how well-designed they are. Further, an MMO usually needs to give the illusion of being "infinite" - you can keep playing forever - to have much appeal to people. That makes them MUCH harder to design and repair. Good single-player games are quite a bit simpler.
The point is the current system gives devs an incentive to focus their time on developing the next ex-pack or game rather than improve the existing product, because that's their only potential source of additional revenue, and I don't like that fact.
But you are supposing that they would do such thing. And why would you subscribe for a game that has a life expectancy of 6 months (according to some companies, because obviously no one plays the same games for so long... just take a look at Master of Magic) when they will move to the next part of the franchise? (one or two exp packs later if truly succesfull or The Sims)
If the fee is $10/month, and it's a good game and I stay subscribed for 6 months, the developer gets $60. If it sucks and I unsubscribe after one month, the developer gets $10. That's a lot better to me than a model where the develop gets $40 regardless of how much the game sucks.
I'm not saying a subscription model would be guaranteed to fix everything; I'm saying I'd like to see some developers try it. I know I'd pay Stardock $X/month to play their whole catalogue and be in their betas, and I think a lot of other people would too.