He was fired for doing exactly what ownership asked him to do. That was even more true in Detroit. I mean, Detroit has struggled when they got old - but they made the World Series, an ALCS and were relevant and competitive to some degree or another.
The big crime Dombrowski made (in retrospect) was not adding players in 2018/19 offseason - but clearly ownership did not have an appetite for that. The big drivers of the dropoff in 2019 were dropoffs by Sale and Porcello - and the fickle realities of middle relief pitching on Earth.