I don't think zn is saying there's 2nd contract cap or limit: to me it sounds more like he's saying there's a pattern of 2nd contracts going up every year that relates more to the increase in the cap than it does to how good QB 'X' is compared to Wilson.
The cap is projected to go from $188,200,000 in 2019 to $200,000,000 in 2020. [
overthecap.com]
That's in increase of 6.27%.
Applying that increase to Wilson's contract: QBs who sign their 2nd contract in 2020 are probably going to be in the 4 year, $148M range just due to the way 2nd contracts for QBs have historically behaved.
Goff, Wentz, Winston, and Mariota are all FA's next year coming off their rookie deals (although the Rams and Eagles can (and likely will) invoke the 5th year option on Goff and Wentz).
Winston and Mariota are both playing on the 5th year option in 2019 for $20,922,000. When they sign their 2nd contracts in 2020 they'll be around 4 years, $148M.
AlbaNY_Ram