Siemian was drafted in 2015 I believe. Even then I don't believe anyone would suggest he is doing particularly well in Denver.
Wentz started out hot against what you guys would call "garbage wins" in Cleveland, Chicago, and Pittsburgh for 5 tds 0 ints, but then goes against Minnesota, Dallas, and NYG and gets 2 tds 4 ints and just finished an awful performance against Seattle.
Dak Prescott has been doing well, but as has been stated, has the best Oline/RB combo in the league by far. Even if we accept him, that is one QB.
If we go outside of this year, you won't find much supporting your argument either.