Todd Howard had an interview after the Bethesda E3 presentation:
https://www.youtube.com/watch?v=v-3X04jwJ0U
He goes over a lot of this info. He mentions that mod support will probably come to Fallout 76, but it won't be at release. He also confirmed that it is online only, and even if you only play solo you will still run into other players. He acknowledges that griefing might be a problem, and says they're still tuning that.
I'm not sure that Bethesda is cashing in on the Battle Royale craze. This looks like its been in development for a long time, before PUBG and Fortnite came to prominence.
Personally, I have zero interest in battle royale or MMO style play. This one'll be a pass for me, though my brother has expressed an interest in playing it with me. If he pushes me hard enough, I might get it.