The reason so many of us have been hand-wringing over the lack of a true WR2 (or anyone even approaching that level of talent) isn't so much about how many receptions that player can have. When you lack a truly dangerous #2, it allows your opponent to smother the WR1, double team him, slide additional safety help over to limit him. There is a reason why Terry McLaurin has never had 100 catches in a season, and it's not because of the QB play. It's because teams prevent him from being able to be open, targeted, and make catches.
Stefon Diggs, even at nearly 33, will eat teams alive if opposing defenses don't respect his talent. That is going to open up things for McLaurin in a way he has not had the benefit of his entire career.
I think that's partially true. I think there are multiple factors to why Terry's been stuck on 1000:
1. The lack of other weapons, as you mention.
2. Idiot OCs for the first Jay, Norv Jr., EB for the first 5 years of his career. ** KOC was the OC for Bill Callahan as an interim coach, but Bill only let him call 7 passes a game, so that doesn't really count. For those Jay defenders, even you you say he knew what he was doing, he only lasted 6 games into Terry's rookie season before Ol' Man Callahan installed the "Run and Run and Run some more" offense so they could lose games closer.
3. QBs: The only QB who Terry played with until Daniels who could be considered good was Alex Smith for whatever it was, 10 games. Otherwise, Keenum, Haskins (RIP), McCoy, Kyle Allen, Taylor Heinicke, Garrett Gilbert, Carson Wentz and Sam Howell all just absolutely blow chunks. And I left off Fitzmagic because he played 6 snaps or whatever then broke his hip and retired. I'd argue there isn't a viable starting NFL QB in that group who should be starting games for any team at the point they played for Washington. Wentz was viable with the Eagles early in his career, then lost it for whatever reason. But he wasn't viable by the time he got here.
Terry has been about the only functional part of dysfunction since he got here. And it's everything, not just one thing.
Though I don't disagree getting more weapons will help pull coverage away from him. I don't necessarily believe that weapon has to be the other WR spot, I think it can probably be any other weapon, slot, TE, RB, even QB though that has it's other issues. However, Diggs is the second best weapon on the team right now, so it checks the box and I'm happy we have him. Well, third best behind Daniels. But again, we don't want him to run TOO much even though I want him to run more than most.