They can restructure, cut or re-sign players(Martin) as needed to increase cap space. Don't need to be an accountant to figure that out.
It's beyond comprehension to also let your best passrusher, in the prime age of most passrushers careers to simply walk away, too. Besides finding a franchise QB, getting a legit LT and DE is up there on the importance list, hence why great DEs get paid the way they do.
Not like we have a plethora of quality DE's behind Lawrence waiting for more PT. Besides, the franchise tag can be used as a temporary method in order to get a long term deal established without having to worry about a team poaching him.
His cap hit is 17.5mil next year, #1 in the league for Left Tackles for 2018
https://overthecap.com/position/left-tackle/2018/
I do agree trading him is silly, even with the recent injuries. Even when hampered, he still is a top LT in the game. Even if they did want to trade him, due to restructures, we would owe him more money if we traded him before June 1st. (18.3mil dead cap)