Yeah I can forgive them but...if they thought it was a mistake, why didn't they think that before they released it?
They were making a Fallout game. They claimed (although the claim looks pretty shaky in hindsight) that they played FO1 and FO2, and understood the "Fallout feel." They tried to do a "Fallout-style" ending and found out that they really
couldn't. So they tried to fix their disaster of an attempt at a real ending with a disaster of a retcon (Borked Steal DLC) that wasn't much better, except for the actual "we can keep playing" part.
Basically, they didn't realize it was a mistake until their really devoted fans started kicking up a fuss that Bethesda had released a game with
"OMG an END WTF!!1!!"