Folks really fail to appreciate how little experience Mac has. There is a perception that rookie development is accelerated, and maybe in some cases it is, but most players (especially QBs) don't reach their mental/physical peak/prime until probably 4-6 years into the league. That's not to say they can't be really good players before that, but it's not going to fully come together for any player, especially a QB, until they have enough accrued playing/game experience, and likewise for strength/conditioning as it relates to physical maturation.
One season + two games is very little time. If he looks how he does now in year four, then it's fair to legitimately question his ceiling. But I'd suggest giving him three full seasons before writing him off (hopefully we see incrementally improvement in the meantime, which I believe we are).
If memory serves, Brady was selected in the 2000 draft, and didn't become a full-time starter until 2001. In my opinion, Brady was a good, not great QB in 2001 and 2002. By 2003 he began really polish/elevate his game to a consistently very good/great level, by 2004 he was elite, and by 2007 he was a special player and already historically remarkable.
Point being, it wasn't until 2003/2004 that we truly realized just how good Brady could/would be, which would've been his fourth and fifth seasons in the NFL, respectively.