During QuakeCon 2018, IGN spoke to Bethesda's Pete Hines and he told them that just because Fallout 76 won't be available on Steam, does not mean that future titles from the publisher won't come to the platform.
“We did not announce ‘all future Bethesda games will not be on Steam,’ That is not what we said. We said ‘this game will be available exclusively on Bethesda.net.’”
At this time there is no confirmation on whether or not DOOM Eternal or RAGE 2 will be available on Steam or exclusive to Bethesda.net but Hines did mention that Fallout 76 might eventually be available on Steam.
“Is it possible? I guess, but I honestly couldn’t give you any guarantee one way or the other on whether it will or won’t.”
The Elder Scrolls Online also released on Steam after it launched exclusively on Bethesda.net so there is hope for people who don't want to use Bethesda's own launcher, if they can wait.
Hines also elaborated a bit on the reasoning behind having Fallout 76 be available exclusively on Bethesda.net.
“We feel like the best way for us to provide the best experience and service to our customers is to be dealing with them directly, and not through someone else.”