We all know in the NFL that time is money. Thus, teams don't have the patience to groom players - especially QBs for 3 years. Your essentially saying, we're ok with sucking for 3 years as we develop him.
Also, QBs brought into this league as backups never get the opportunity to start extensively. They were brought as a backup to be a backup. He'll always be a backup. If backups are given the opportunity to start, it's usually via trade and into a bad situation where most QBs would fail. The odds are just stacked against them from the get go.
If the Giants see Nassib as having all the skills necessary to be a good QB - with time, i don't see why it's far fetched to think that he could be Eli's replacement. He has all the time in the world to sit and learn. We have a solid front office, little turnover, and should have a decent cast as we draft well. Heck, Sanchez came in and went to 2 AFC Championship games with a solid supporting cast.