Rosen has done nothing to warrant any trust in building around him and passing on Murray unless you think Murray is not good at all.
So Murray it is.
QB is the most important position so you always need to prioritize getting the best QB possible no matter what your needs are.
Even if you draft for need you don't even know if you actually fill the need. The draft has been a crapshoot also. If you spend top 10 picks on defensive linemen you have no clue if those guys are even going to be better than 3rd round picks.
I'm going to stick with my emulating the Rams theory. The Cards got their Pac-12 QB like Goff. They got their perfectly coiffed, young, innovative offensive HC like McVay. Now they're going to add a beast NT to shore up the defense.
Granted, the sequence and timing of everything won't perfectly match but I really think they'll build around Rosen.
People bring up the Rams because they are a good defensive team yet looking at their draft history:
their first two picks in 2018 were a Tackle and Center,
their first 2 picks in 2017 were a TE and a WR,
their first 4 picks in 2016 were a QB, 2x TE and a WR
their first 3 picks in 2015 were a RB and 2x Tackles.
The last time the Rams used a high pick on defensive player was Aaron Donald at 13 in 2014 and Michal Brockers at 14 in 2012.