On the flip side, you've got the EVE Online model. Expansions are released every 6-8 months as "super-patches". Like every MMO it's mandatory to be playing on the latest version, but everyone gets it for "free".
It's pretty simple to avoid the problem of having players scattered across multiple content sets and levelcaps: stop bleeding your players dry. What are subscription fees paying for if not ongoing development of additional content?
IMO you should either charge for expansions or charge subscription fees - but not both.
Yeah, it's hard to make the business case for leaving money on the table - with every expansion comes change and change much more than the expansion cost is really what is causing the gating effect.
Treating your customers as customers rather than cash cows engenders positive feelings toward your company, and ensures continued subscription. There's your business case.
It's pretty simple to avoid the problem of having players scattered across multiple content sets and levelcaps: stop bleeding your players dry. What are subscription fees paying for if not ongoing development of additional content?
IMO you should either charge for expansions or charge subscription fees - but not both.