In 2014, BB the GM drafted Jimmy Garoppolo in the 2nd round, and explicitly cited Brady's age and contract status. Then in 2016 BB the GM drafted Jacoby Brissett in the 3rd round. Both picks looked really good when Brady was forced to sit for 4 games in 2016.
Of course, Brady defied time, and finally Bill was forced to choose between Garoppolo and Brady in 2017. And he undeniably made the right choice, albeit one that was going to lead to the team still needing to find a replacement in the future. And that choice of Brady was coupled with the Patriots going all in on 2018 and 2019, a choice that was universally applauded on this forum.
Still, the Pats drafted Etling in the 7th round in 2018 (a pure flyer, as any 7th round pick is), and Stidham in the 4th in 2019. But there were also few chances to draft a replacement in that period. The Pats traded most of their 2017 draft capital for more immediate needs, and Mahomes was out of reach for them anyway. They possibly could have picked Lamar Jackson in 2018, although they instead focused on immediate need and drafted Michel and Wynn. Maybe in 2019 they should have drafted Minshew instead of Stidham, but that's really going out of the way to find fault. In the 2020 draft, they probably should have drafted Jalen Hurts; fortunately, they did not draft Jordan Love, who is likely going to have an even worse career than Mac Jones. But a lot of teams passed on Hurts as well.
There just was no "easy" solution to find a replacement for Brady, no matter how many times we wishcast it otherwise. BTW, the bolded in Felger/Mazz nonsense. The team went all in and started 8-0 until the defense tired and the Antonio Brown and Josh Gordon experiments went awry.