Well, they were in a SB, which is as close to winning a SB without doing it as it gets. That means a ton to most fans (or at least it should) because getting to a SB is extremely, extremely, extremely difficult. It's certainly not a negative on anyone's resume.
The only team that's really had more success in the last three years than the Rams is the Chiefs, Bucs, and maybe the Patriots. And notably, two of those teams are operating under basically the same philosophy, i.e. they're spending tons on veteran players, and trading away draft picks for them.
"Sustainable" is subjective. If you want to be relevant for a decade or more consistently, I agree with you. If you want to be at the top of the league for a 3-5 year period, possibly win a SB, and then fall back down to Earth, then so be it. I'd take the Rams last three seasons over the Ravens last decade (pretty much post SB win) any day of the week.