An upgrade from Brock yea, but both are downgrade from Tannehill.
Considering Brock has been a slight upgrade from Tannehill this year that's not possible.
Here is what Dak would have to do to match Tannehill's career at this point...
15 wins 27 losses
Passing
970 of 1580, 61.39%, 10,776 yards, 6.82 YPA, 59 TD, 49 int...81.18 QB rating
Rushing
79 att, 270 yards, 3.41 ypa, -8 TDs (literally have to somehow throw away 8 TDs), lose 9 fumbles
And Dak would have to...
Have a 51 to 58 TD to Turnover ratio...TD (TD Pass + TD Rush); Turnovers (Interceptions + fumbles lost)
Have "1" GWD...lol (Dak has 10 in 40 games; Tannehill has 11 in 82)
Somehow undo the playoff game he has already been in...24-38, 302 yards, 3 TD, 1 int, 79.1 QBR, 103.2 QB rating
Somehow throw away a division title
Miss 23 games and counting
That's just to get on equal footing with Tannehill's career.
At this point Dak is the same age as Tannehill was 8 games into his second season.
I may be crazy, but I think Dak can surpass the Tannehill standard by year 7 of his career.