That is conventional thinking, but not quite correct. The Rams signed and lost a DT like Suh....lost John Johnson and others, but they also received compensation picks for these guys the following year, The Rams have been excellent at finding guys in the later rounds, along with UDFA's that go on to start for the team.
There are a number of recent examples on the Rams roster where this tactic isn't working as well. None of the LBs they've tried have been close to as good as Littleton was, and the Rams have tried a number of guys.
One of the reasons the Rams aren't as good defensively this year is that they aren't getting as good results at DB positions as they did in the past. Long isn't good, Rochell is only ok, and Williams wasn't playing nearly as well as he did last year. Troy Hill was kind of a glue guy for the CBs and losing him has hurt.
Fuller hasn't really gotten better, and Rapp is pretty inconsistent. The Rams are definitely missing John Johnson.
Brad Holmes, who the Lions filched this last offseason to be the GM (I believe), was instrumental in finding these guys and I worried what effect it would have on the Rams system, but so far it seems to be good. As a consequence, the Rams are the 5th youngest team in the NFL.
The Rams have lost a ton of coaches of the years and we're seeing some of the effects. I think Staley is one of the 10 best coaches in the NFL already, and Morris isn't nearly as good. I know from watching the Cardinals vs. Rams I could tell a big difference in how the Rams D played the Cardinals.
The Cardinals are even younger than the Rams right now.
You probably didn't notice that Joe Noteboom has exponentially progressed as the Rams future LT after Whit retires. He has started in several games the last couple of years when Whitworth was injured and the transition was smooth. Whit has taken Boom under his wing in the offseason, and the kid has improved in strength and technique.
In other words, our future demise is greatly exaggerated.
Noteboom has been a nice find for the Rams and he's lucky he's had the chance to sit back and develop. He didn't look great at first, but he's developed into a good starting lineman and the Rams are lucky to have him on the bench.
I'd say the Rams have a few year window right now. They can kick the can down the road and bring back most this roster, but they will have to leverage future flexibility. You mentioned they can create some cap space by extending Stafford, but that will have future ramifications. As much as I like the move for the Rams to trade for Von Miller, he is 33, and is a rusher that relies a lot on his speed rush. That's a guy that could fall off the cliff any time now.
And damn, I didn't even notice that Noteboom is a free agent. Some team might come in and throw 16 million plus at him. He has good enough tape to warrant a big contract. Noteboom, Corbett, and Allen might all get real good contracts on the open market. Even Edwards should as well. Darrious Williams should command a top contract too. Lots of guys to resign on 7 million and to keep a 33 year old pass rusher.