It's been discussed here. The original succession plan was Jimmy Garappolo, but Brady broke that plan by continuing to play at MVP level.
Team went all in on 2019, signing Brown and giving Brady a one year voidable deal. Made sense at the time as it was obvious that it was going to be Brady's last season with New England. But the WR corps went belly up for various reasons and then Brady moved on. Pats had no cap space to bring in any good free agent QB's or even make a trade, so were left with the scrap heap guys, which included Cam. Also, the timing of the 2020 offseason was altered by CoVid, so the June signing is not as late as it seems.
And rather than throwing desperately needed draft capital at a marginal veteran, the Pats went in on the 2021 QB class instead. Betting on Mac was not the huge mistake some revisionist posters in this thread are making it out to be; Jets made a far larger bet on Zach Wilson. And the Pats hedged the bet with Zappe. And the defense is better and younger than it was in 2019 (that group had a great start but aged out quickly in 2nd half of the season) or 2020 thanks to the additions of Duggar, Barmore, Uche, Judon, the Jones group, et al.