Rice also had Joe Montana and Steve Young throwing to him. Megatron is great at running routes. He's not Rice, he doesn't have Rice's hands, but he's got insane athletic ability.
Fitzgerald was in the mold of Jerry Rice, sick hands perfect route running, etc. But when he was in his Prime vs Megatron, Megatron was still better. Megatron is the best receiver I've ever gotten to see for a reason. Rice may have been better, but I think you're talking about 1A vs 1B.
Rice played in an era where defenders could defend. Teams ran the ball. Officials didn't throw flags at any sign of contact or roughness. Defenders could knock his head off if he went across the middle. Johnson plays in an era where the rules are geared toward big offensive numbers. Defenders can't play defense, his QB throws the ball damn near every snap and he can't get his block knocked off after he makes a catch.
Offensive numbers in this era are highly inflated and as of right now, Megatron is on pace with Rice. He's going to have to have to do what he's already done in his career and do it again for another 7 years. After another 7 years of being the best WR in the league, he needs to then be a solid top 10 receiver for the last 4 years or so of his career. Only then could you make a 1A and 1B comparison.