Nice post. My perception is clearly different from reality. Maybe it was the drop off from 50:8 to 21:11 and the disappointment of finally getting to see Brady come to the Bay Area and instead being incredibly let down. I concede this point, that Cassell did come on strong at the end and played better than I remember, although his QB rating was roughly 30 points below Brady's the previous year with a very similar offense. I guess a lot of it is subjective..I just thought he never really looked the part and had some pretty big limitations.
I certainly agree with you that Cassel had obvious limitations, and I think we saw a lot more of that once he left and took over his own team(s).
I do think that the 2008 season showcased his strengths quite nicely though, and as dumb as it sounds now, there were a decent amount of posters and mediots who felt that he'd bring in multiple high round draft picks.
The same logic applied in terms of it being a QB-needy market, etc. Hell, some even called for him to replace Brady, and if I recall correctly, early 2009 was the period where Curran's (incorrect) story broke about Brady's future being in jeopardy. He had developed infections in his knee and many believed that the waters were murky in terms of his recovery time. There was lots of chatter about needing another surgery. As mentioned, some were concerned that he'd even return at all.
The fact that Belichick traded Cassel in early March during this time of uncertainty spoke volumes about his confidence in Brady and his ability to return, but I believe the situation may have actually helped drive the value of Cassel up even higher. Ultimately, we know how that turned out, as most of us were surprised and disappointed that he only brought back a 2nd. I think Garoppolo will definitely bring us more, but I'm not as sure as some regarding how much more that will be. Like I mentioned in another post, personally, I will be happy with a first round pick. Either way, I think we're in good hands, so that's all that matters. In the meantime, these debates help to pass the time of the offseason, and help to deal with 3 AM insomnia.