Well, Skyrim is almost certainly over a million already, and while Doom's numbers may not be chart-scorching, it should be remembered that porting an existing game is a hell of a lot cheaper than making one from scratch.
It's common on forums to act as though everything under a million is a flop, but Doom's Switch port may only have needed, say, 200k to turn a profit, with everything beyond that being gravy. Since these ports are being handled by external dedicated porting houses (Panic Button/Iron Galaxy) it's not like they're tying down significant resources at Bethesda, so they make a profit with little risk or investment, the middlemen get to make a tidy living, and Switch owners get more games. Everybody wins.
The simple fact that Doom Eternal is coming at all suggests their efforts thus far have born fruit, if not they would have abandoned the platform by now.