The problem with rookies is that they have proven nothing. It takes a couple of years to know what you have in them. Trading for a vet means you know what you are getting and what you aren't, i.e. either they fit the offense or they don't. Take someone like Penix and the Rams have a whole year to coach him up. That said, Penix is very close to a first year starter type of rookie.
I would like them to draft a QB and have him at an affordable price for 5 years, but that means a learning curve and all the issues that come with developing a young QB. As we have seen McVay isn't the most patient coach when it comes to developing players. When it comes to QBs they are either his guys or they aren't. If they draft a QB it's not going to be a developmental type of guy IMO and the vast majority fall into that category.
Regardless, next year's draft looks to be a very interesting one.
