Well, Gurley is guaranteed a bunch of money. He's safe. Suh is gone - the Rams won't have enough to sign him and Donald. But there are others who would be gone with signing Donald to that huge contract.
Looking at Spotrac, with Havenstein's contract the Rams will have about $41 million in cap space available. If Donald signs a contract and has a cap hit of $24 million, as many speculate, that would leave $17 million. "But wait" some will say, "it can be backloaded". Well if Donald agrees. And keep in mind that in 2020 Goff's cap hit will likely be around $25 million, and likely over $30 million in 2021 the way QB contracts are going. (5th year rookie contract tenders are priced as if they are transition tags. In 2018 for QBs that would be $20.9 million - but there was a number of very large QB contracts handed out this past offseason, and Aaron Rodgers will get an even bigger contract soon - so this number is going to skyrocket for 2019, and be even higher in 2020.) So there isn't really a lot of room for backloading. Oh, and add in Gurley's extension, which starts in 2020. So no, backloading will not be a savior.
If Donald's cap hit is $24 million, then again, that would leave the Rams with $17 million or so available (keep in mind that Spotrac is already assuming a cap increase of over $10 million, btw). Now, the Rams will need to replace one of their starting cornerbacks, a backup cornerback, their star free safety, Suh, an All Pro 2nd string guard in Saffold, their other starting guard, various backups - oh, and I suspect they will want to get a veteran backup QB. Mannion is a free agent, and will be more expensive. Allen is less than inspiring. I expect them to sign a vet backup and draft someone too. They also have holes in places like OLB and TE. Plus, of course, various backups needed. A few of the holes will be filled from within - but not all of them, and if the best backups are moved to starter, other backups will be needed. Not everybody who is cheap will work out, so the Rams will need some money for late signings. In addition, every late round draft pick and desirable UDFA will have signing bonuses - if they don't make the team, that money will count for 2019 - not necessarily a fortune, but it has to be counted too, and likely close to a million. And the practice squad costs over a million in cap space too.
Maybe the Rams can go cheap and fill all the holes - but more likely some players just won't work out. Oh, and injury replacements count against the cap too, as do players on IR. Oh, and players on the practice squad.
I want you to demonstrate with all this showing that money would be very tight, that the Rams would actually have cap space to use more than cheap rookies to fill holes. Given what cornerbacks cost, unless they use a high pick they are unlikely to replace Talib (and high picks have large cap hits). Suh won't be replaced - again, unless they use a high pick and get lucky. It will be hard to have a quality OL with this cap space. Unless they use a high draft pick and get lucky - but they only have one first after all. Free safety is likely to decline. Basically, by this point, you need to show that with signing AD to a mega contract the Rams won't likely have multiple holes they really won't be able to fill, or you are just being foolishly optimistic.
Wade/Johnson do have two young vets who will be non starters( if AD were here) ,who do have what it takes to be an effective replacement for the vacancy @ this post. vet's Ethan Westbrooks & Dom Easley. In addition both starters NT Ndamukong Suh & DE Michael Brockers have lined up in the 3 tech & played in the NFL very successfully. Just so happens Westbrooks & Easley have played & started in the NFL @ Suh & Brockers assigned posts. In addition rookie John Franklin-Myers & Sebastian Joseph-Day stand @ the moment to make this 53 player roster & figure into Rams future plans. 
