I should rephrase myself. Great "potential" QBs come out every draft. There will be another Mariota next season. And the season after that. Also, recent history, at least for the Rams, has shown that our first rounders make an impact in their first season. Brockers, Austin and Donald were all important pieces to the team their rookie seasons. And they made a difference. If we took Mariota at 10 and missed out on a HOF guard or tackle because we reached for a potential franchise QB, it would be a fatal mistake in such a crucial year.
I'd argue that even great "potential" QBs don't come out in every draft. But, even if they did, that doesn't mean the Rams would have a shot at one. This is why teams trade so much just for the opportunity to move up and select these guys (plus their overall impact).
Brockers, Austin, Donald, and even Robinson played. But, the only player to make much of an impact as a rookie that perhaps a stopgap couldn't compete with was Donald.
I don't know what Mariota will turn out to be, but if the Rams view him as a potential franchise guy, they'd be nuts to pass on him. The impact of the position is overwhelmingly greater vs. one OLineman who can do nothing without 4 other players with him on the same page. Besides, I can't recall a HOF G or T leading his team to a Championship. But, I can think of several very good OLines without a HOF player.