No the Niners let Montana go because he was starting to slip and they had a successor lined up. The Packers let Favre go because they’d drafted Rodgers but he only had 1 year left on his rookie contract and they needed to know if they had their QB of the future or not and Favre was screwing them over with the “will I/ won’t I”, the team chose to no longer be held hostage. The Pats let Garappolo go because they don’t feel they need to replace Brady yet, and they were going to lose him anyway (contract ended this season, no way he was going to sign for low money to stay a backup). By the time Brady falls off they’ll have a backup, remember he WASN’T supposed to be the guy to run Bledsoe out of town, just a 6th round draft pick nobody expected anything from. The Pats know talent.
If you think the Pats knew what Tom Brady was going to turn out to be and they craftily waited until they could get him at super great value in the sixth round then we'll disagree. They got lucky. Sometimes that happens. Their odds of getting a great QB in the sixth round again is essentially zero. Half the teams in the league have spent decades trying to find a true franchise QB. Only a handful of teams really have one. The odds say that when Brady retires his replacement will be a gigantic falloff, immense drop. Who could argue that? They could draft the next Rodgers or Russel Wilson or the like but odds are more likely that they struggle to get a guy who better than average. Garropolo might have been that guy, they might have hit gold there (jury is still out but early returns look good). But the rumor is that Belichick wanted to promise Garropolo he'd start next year, Kraft said "no way, trade him".
I make no bones about it, I'm sick of the Patiots. I look forward to watching the Bills, Dophins, and Jets getting revenge for 20 years of beatings. I hope it happens soon. I'm not objective. But I do think the Pats have a real QB crisis coming soon. Just my opinion.