You're mistaking full-time starters with WR groupings. This team doesn't have a Torry Holt/Ike Bruce combination that plays all snaps. This team utilizes different WR groupings at different times. There won't be a receiver on this team who plays every snap of the game. I understand you want to jettison what you consider to be dead weight in Pettis and/or Givens, but then all you're doing is limiting what the OC can do at different times AND replacing experience with rookies. I mean, how many times do you want to field an offense that has a year of experience?
This is a pretty decent roster projection even if there might be disagreement on the fringes (e.g. maybe 6 CB and 4 S rather than 5 & 5).
http://espn.go.com/blog/st-louis-rams/post/_/id/9465/rams-projected-roster
Givens, Pettis... hmm... neither did well without Bradford last year. Week 1 with Bailey not counting against the 53, I have to imagine both will be on the roster unless somebody steps up to shock us during camp or pre-season such as Blake/Veltung. After Bailey comes back, I can see the math catching up to one of these guys. Givens seems like the 3rd in line for small fast guys behind Tavon and Bailey. Pettis seems like 3rd in line for big physical possession guys behind Britt and Quick (and Cook), even if I still need to see the 2014 versions of Britt and Quick in live action. Being 3rd in line puts them on the fringes where special teams contributions will go a very long way to determine who stays and who goes.
Here's the real deal, tho. In the VERY unlikely event that everyone is still healthy come week 5 when Bailey returns, yeah, I can see Pettis getting released. But anything can happen to save that spot. One of the top 5 WRs can be injured. JLong playing healthy and we release OL instead. The top 4 CBs can be playing lights out and so we release a 6th CB instead. We might try to stash a young guy on the practice squad hoping they clear waivers to make space. The roster is really much more fluid than how we fans see it.
In 1 year (2015), if Britt (re-signed) and Quick and Tavon and Bailey prove themselves in 2014... yeah, things get really tough for one of Givens / Pettis.