Could be and is are two different things. For example, Malcolm Mitchell could be a solid #1 WR in this league some day based on how his rookie season is going. Saying it would be ok to trade Edelman in the offseason because Mitchell could be the #1 WR to replace him is overrating him.
We don't know what Garoppolo is. At this point, he is only potential. People are talking about about him as if he is definitely going to be a top 5 QB in this league. That is overrating him because there is nothing definite about it. For all we know, he is just as likely to be back up in three years as he is competing for an MVP title.