On paper, losing Kimbrell and Kelly was pretty significant, but yes, that team looked very good, despite those losses, on paper.
Declining production from returning players, especially those not past prime was unexpected, and that sort of thing happens, often, just as a bunch of players can all gel at the same time.
Often, the manager and or GM are blamed for those declines, and I suppose some of the blame can be placed on them for not planning on some declines better, but there was a change at the top before 2019 even started. Yes, a lot of money was spent of bringing back Sale, Bogey and Nate, but basically no money was spent on anything else, and no more prospects were traded after the 2018 deadline. It seems like you have to continually seek to improve on paper, every year. Trying to stay even, usually results in declines. Not replacing Kimbrell & Kelly was asking for trouble, and we got it.
Losing Betts, Porcello, Price and others, after 2019, without spending hardly anything to replace them was furthering the major shift in philosophy.
I don't think the 2019 was as close to teh 2018 as we think, and the 2020 team was a shell of even the 2019 team.