As for ways to fix this? I can think of one strategy that, in a certain sense, would try to hybridize the two models:
1. Gather a team of writers, say 5-8, who will be paid for a block of multiple adventures, not just a single one.
2. Have the team agree on an overall, between-adventures set of events that unfold. Some of them will be actual adventures in the block, others will be general current events.
3. Each author makes a series of adventures that loosely link together--you don't have to do every step in the sequence, but if you do, there are fun bonus bits.
4. The authors are paid in part for the collective sales of the whole block (or phases thereof), and in part for their personal contribution. This way, even if one specific adventure just doesn't sell as much as the others, it doesn't result in that author losing their shirt, but you also don't take away the success of someone who wrote a wildly successful one.
If this works, it approximates the best of both worlds. You have opt-in adventures across a span of levels (so every DM has something they could get value out of), while still having a consistent "background" for things to play out against. You get a light touch of the "Epic" linking story between things, but you don't have to engage with that if it's not interesting to you.