While I agree that Robinson is an excellent player and would improve the offense significantly, I'm not in favor of adding him.
Successful QB's are generally ones where the team builds around them. I'd like to see us pair Tua with an elite WR like Chase, Smith or some other top young WR#1 prospect. Then we could play Parker opposite him and watch both players production go up. Robinson would artificially create a bridge bubble to Tua needing to break in another young WR later. IMHO, the ideal time to do that is now, when both are hungry to get the extra reps after practice, eager to soak up the knowledge, and can provide confidence to one another to build a bond that becomes great for a decade plus.
Nothing against Robinson, but it would need to be to replace Parker, and I'm always leery about FA WR's because so many bust when changing teams. Tua needs the one guy with whom he has built chemistry, Parker.