I can't see a way to frankly assess the quality of a game on its commercial success, but let's at least not pretend the franchise's popularity is based on its later installments - that puts the cart before the horse imo.

Interplay going bankrupt is the reason Bethesda owns this IP. FO3 wasn't a bad game, but it started development before their involvement. Everything Bethesda made on its own has been increasingly in their own simulationist, environmental style, which can be fun but isn't a good fit for the highly novelistic style that made it popular to begin with.