Very easy.
https://www.nfl.com/news/nfl-qb-ind...-starting-quarterbacks-at-the-end-of-th-x3120
Dak is simply a better QB than Purdy.
https://www.pff.com/news/nfl-highest-graded-quarterbacks-2023-nfl-season
Dak finished the 2023 season as 2nd in the league MVP race. He finished with the 3rd most passing yards (4,516), most passing TDs (36), one of the lowest INTs totals (9), highest completions % (69.5) and the highest passer rating (105.9) amongst all NFL QBs in 2023.
https://www.nfl.com/stats/player-stats/
Dak accomplished all of this with a severely limited offense. Only 1 legit threat at WR, no threat at RB, not much of a running game at all, only a 2nd year TE and a newly revamped offense where he started off the whole month of September slow and with very low numbers due to McCarthy trying to go run heavy and rely on his defense while Dak and his very limited offense got acclimated to the new offensive playbook (new Texas Coast offense). If McCarthy had allowed Dak to start the season simply throwing a lot during those first 4 games of September, Dak would have easily passed for over 5,000 yards and had much higher passing numbers overall.
Remember, all of this accomplished in 2023 after starting the season very slow by design the whole month of September and playing on an offense with very limited weapons and no running game whatsoever. You plug Dak on the extremely talent loaded 49ers team, they would have won the Super Bowl. Guaranteed. To deny this is simply hating at the highest levels.