That simple statement sums up exactly why many do not want to invest what would likely be required in terms of both money and time.
The QBs you mentioned are widely seen as the best in the game today, precisely because they have skill sets (three distinctly different skillets) that allow them to do things that Tua hasn't the tools to call upon when it's the difference between a win and a loss.
For all the OPs time and effort to make a Brees comparison, the fact still remains Brees was an outlier. Expecting a similar career path is a bad bet, not even getting into the one Championship in 20 years issue. I mean are we now arguing that we should hitch our wagon to Tua for 16 more years in the hopes he can repeat Brees' very unlikely HOF path?
Too many get labeled as "haters". I don't really think there are many who actually hate Tua. He is, by all accounts, a good and humble guy. Most of his detractors simply do not believe physical abilities are on par with the best in the league today. That's a wholly valid, non biased criticism that, to use a Big O term, "blind a$$ fans" refuse to accept as a rather large shortcomming.