There are certain things that I think people will pay for when it comes to monthly fees.
Persistent world that has regular updates and new sections added are a must for games with a monthly fee. A centralized server area that is always available and hack-proof, or at the very least has version/mod control. So to play with a friend with 17 mods on you also have to have those mods on.
Most MMOs are little more than FPS games played in third person. The differentiation is leveling a character. With the mash up of the two genres happening in varying degrees already (leveling your FPS avatar) it's not that much of a leap to have pay-to-play FPS games if they are as stated in the OP, very polished and addictive.
EA and Activision are battling for the designation of evil empire when it comes to squeezing consumers, enforcing absurd DRM, generating negative press, and pissing off the gaming community. It's no surprise that these companies would want to milk an ongoing fee-structure for games. It's the best of all DRMs, and that seems to be at least 40% of their product; DRM. Enforcing, oppressing and generally being dicks.
It's too bad but that's where I see this going. Not if a pay-to-play FPS can be successful, but a matter of when it will be forced onto a highly desired title.
I think the other enormous hurdle for evil empire gaming inc to overcome is the extreme price point that many other MMOs have had. Why is the 'standard' $10 minimum per month? If the shill was $2.99 per month I could foresee a lot better adoption and much less complaining. Then just like cable companies, cell phone companies, and every other consumer-hating industry out there they can nickel and dime for extras and micro-transactions.
The saying used to be 'the customer is always right' ... it's been changed in corporate society to 'the customer always has at least one more dollar they could spend with us'.