I think part of what we're seeing right now is the expense of creating new content. One little DLC micro-expansion is almost as expensive as an entire expansion might have been 10 - 15 years ago. The cost of making games has skyrocketed, and is now an order of magnitude more than it was back then.

So… they create 1/10th the content of an old-school expansion, sell it for 1/3rd the price, and hope they can sell it to 3x more customers than they used to sell full-fledged expansions for, maybe?

I dunno. But I think that may be part of the explanation.