I prefer to see some evidence before touting rookies as upgrades over very good NFL players.
We have seen both Johnson and Hill play in both Staley's zone-heavy scheme and in a more conventional man/zone 3-4 hybrid scheme. Johnson and Hill are both scheme dependent, i.e. zone DBs. They lack the speed to play outside of those schemes. With Burgess and Rochell the Rams now have DBs with the speed to play sideline to sideline or matchup with better receivers. Snead understands that the speed of the game is increasing and team speed is very important now more than ever. Hill and Johnson are examples of good enough type players. Use them within their limited skillset and they can be productive. Outside of that limited box not so much. Both of the aforementioned players are at their peak now and that is a problem for a team that needs to be more.
Rochell and Burgess have much higher ceilings. Yes, it needs to be realized, but the fact remains that potential is there with them whereas it isn't with Hill and Johnson. If Morris wants to play a single high alignment he can with Burgess. With Johnson's speed 4.61 he would need to shade to one side and play shallower. Burgess has true sideline to sideline speed at 4.46 allowing him to play deeper and not favor one side or the other. It may not seem like much of a difference but it is. People say with Ramsey on one side what's the problem shading to William's side? That assumes Ramsey doesn't drop into the StarCB position putting David Long at RCB. That's what was shown as a weakness last year. It's much harder to hide your defensive play when you have limited DBs. Hill couldn't hold up on his own outside. It's much easier for the OC to scheme Ramsey out of the play.
What I like about Morris is that he has proven he can get more with less. Now he has the best roster he's ever had in his career so I would expect this defense to remain one of the NFL's best. What I do expect is that Morris will shore up that leaky run defense. That is why the Niners beat the Rams last year. Staley's defense was pass-centric in its emphasis. That works when your offense forces the opposition to play from behind. I never liked Staley's philosophy of run defense on the way to the QB. The Niners proved that it doesn't work with a team that has both a good passing game and especially a strong run attack.
Bobby Brown, Earnest Brown, and Ernest Jones were drafted specifically to plug that run defense hole. Bobby Brown is the ultimate replacement for Robinson and E. Brown is the early-down rotational guy at the DE position. I see Jones as an early-down option alongside Reeder. If Young has a weakness it's in run defense. So I can see an early-down rotation between Young and Jones. On early downs, Jones would be your A gap plug freeing Reeder to flow to the POA or drop into coverage on an early down pass.
It's clear to me that Snead feels they might not be able to keep D. Williams next year. I see Rochell as his eventual replacement. That could come as early as next year if Rochell shows improvement. It's normally the second year that CBs begin to realize their potential and have their breakout year in their 3rd year. If Rochell follows that developmental trajectory he could be the LCB next year.