I believe the Pats traded Garoppolo on October 30. At that time, the 49ers were 0-8. The Browns would have supposedly sent Houston's first round pick, who were 3-4 at the time. I think the day before Deshaun Watson suffered his knee injury, but I'm not certain when it was announced as to the severity of the injury, in relation to the trade deadline. Also, I don't think anyone saw the 49ers going 6-2 to close out their season.
I don't understand this narrative that Bill traded Garoppolo to a situation where he could succeed. The 49ers were a team that has gotten progressively worse since reaching the Super Bowl. Didn't two of their best young players retire too since then? There was an ugly divorce between ownership and their previous coach, Jim Harbaugh. I don't recall any good players on their current roster that anyone would want.
I do agree that the Pats could have possibly gotten a better deal. I'm not sure of the Pats cap situation next season, and I forget how contract terms differ between a first and second round pick. At the time of the trade, that 49ers pick looked like it would have been a top 5 pick in the second round. The Pats could have received a lot more before the last draft, where the new team could have evaluated Jimmy for a whole offseason and had him play for the entire 2017 season. However, I think you have to severely discount a QB's trade value just before the trade deadline, especially for a backup, who had only started two games and was about to become a free agent. The other thing to keep in mind, is that Garoppolo had to learn an entire new offensive play book before he could even see the field, in a short amount of time. I'm sure the playbook was scaled back to help him out, and possibly made the offense more predictable.
The other thing to keep in mind is the amount of power Garoppolo has this offseason in negotiations. He just became the poster boy of hope for 49ers fans, so they want him to stay. So the 49ers have to either franchise him for one year, or sign him to a contract that probably makes him one of the top 10-20 highest paid players in the league.