The thing is, they have a seven game sample size with Purdy in their actual offense with their actual players and coaches. While the idea of Brady is nice, he'd need to be paid, he'd need to be made to fit into their system, he'd need to not decline further, and he'd not have the kind of control he had in Tampa.
The summary of the SI article wasn't very direct. The underling SF Chronicle article says:
Regarding Brady:
Regarding Purdy vs Lance:
Regarding why this is such a good setup for SF: they gotta pay Bosa...
Source:
Debate it if you must, but the 49ers’ starting QB in 2023 is Brock Purdy
The author is Micheal Silver, a SF Chronicle columnist who previously worked for NFL Media and SI.