So player comparisons are only valid if they're with the last player to be in their depth chart spot? It's fair to compare how Brock has played for us vs how Jimmy played for us, but not fair to also look at how Garcia or Montana played for us? These just seem like odd distinctions. If you find player comparison discussions stupid then fair enough, but they're a central part of sports discourse. We're just comparing performance to start a career.
Saying "Brock Purdy is in good company with how he's started his career" is different than saying "Brock Purdy has always increased the quality of his play game to game." By choosing to interpret that post as the latter, you're taking the term "trending" too literally. It's just saying that Brock has started his career well and is pointed in the right direction, evident by a comparison to the start of Montana's career. That claim isn't contingent on Brock playing increasingly better every game. A 21 group of games is a valid sample to assess average play for the start of a player's career. But according to your logic it would be impossible to tell if a QB breaking the record for highest rookie year QBR had a good year or not because maybe they ended the year with 2 stinkers. Most great players don't end their careers with their best seasons, but that doesn't invalidate their greatness. When people point to Tom Brady holding the record for most passing yards, passing tds, SB wins, SB MVPs, and regular season wins as evidence he's the GOAT, they're not doing a time-weighted analysis, they're looking at the overall picture.