Expansions are intended to reinvigorate games, but they are also what usually convinces me that I am done with an MMO.
When I am into a game, every new thing is exciting. Changes may or may not be good, but I am passionate about them. Revamping major systems is a learning opportunity, a whole new batch of Theory of Fun fun for a spade like me who is an Explorer of design and mechanics. Every set of patch notes is the seasonal menu at a fine restaurant, and an expansion is a smorgasbord of new content.
When I am not into a game, I can coast for a long time. Sometimes that is just downtime, waiting for something new to revive that spark after I have done all the things. Other times, it is a misguided sense of commitment and loyalty, not yet ready to admit that I am done. The current free to play trend lets one drag that out for a long time, logging in for a quick daily to keep a faint spark alive when a subscription fee could force an “is this worth it?” decision point.
An expansion forces that decision point on a grander scale. You need to buy something. Large mechanics are changing. The level cap is probably going up, and if not the population will still be moving to the new areas, so you must follow or be left behind.
And so I look at my emotional reaction. Do big changes inspire interest or dread? You have already made your decision, you just need to recognize it.