Alternate history, NFL scouts don’t sleep on Tom Brady and see his success coming before the 2000 NFL draft, what would have happened ?

Chad Pennington was the 1st QB taken in 2000. The 2nd, Giovanni Carmazzi, didn't go until the 3rd Round, same as Chris Redman.

Cleveland had the 1st pick in the draft so nothing would have changed, historically, had they taken Brady because Browns draft picks always fail.

But Washington had the 2nd and 3rd picks so if they'd taken Brady they may have ruled the NFC East for years.

He would have gone #1 no doubt. But to a worse team such as the Bengals and browns at the time. He would be more limited but could change the face of any franchise.

If his career performance had been foreseen before the draft, he would have been the #1 overall pick.

He would be a backup and out of the league by 3 years.

Scouts didn't underrate him, he was developed into the player he is by Belichick.

there is a "Tom Brady" in every draft, but very few of them get as lucky as Brady did.

Buy a Magic 8 Ball and ask it.

He'd have been picked up by one of the first few teams to have a scout who liked him and thought he wouldn't make it to the next round.  That team likely would have started him almost immediately in a system that he wasn't used to.  He would not have had time to develop and learn as a backup.  Unless the team that got him got a supporting line an offense for him, he would be beat up and out of the NFL after a few years.

Obviously, he would have been #1

Just guessing, but Brady would have probably been picked by the Browns, Bengals, Cardinals, or any number of teams that have been stuck in mediocrity for the past 19 years.  It wasn't a very good year for quarterbacks.  Aside from Brady only Bulger, and Pennington had any significant playing time.  

He would have been drafted earlier.

The Browns would have selected him and ruined his career and he would have faded off into oblivion.