I think cleanup is part of it. I also have noticed as a guy on the DS job family, but who has taken a large interest in SDE work, separating the job families can result in churn. For example, I might think of three model choices A, B and C. C may be the worst of the three, but only very marginally worse. It can also be the case that C is an order of magnitude easier to keep and maintain in production.

I've seen cases where the wrong choice here ends up requiring three SDEs for half a year, where if they gave up a tiny benefit of the best model, they could have done it with 1 SDE in 1 month.