The only benefit of not signing Lawrence to a long term deal right now is if Lawrence falls of the map or gets injured next year. His talent will probably not diminish that quickly but injury is a concern. Odds are greater he gets injured sometime over the course of the next few years than it is specifically next year alone. That's a captain obvious statement, but if injury is such a concern, the most likely outcome is that he remains healthy next year, earns a big contract heading into 2019 and then the chance of injury increases since he will be on a multi-year deal. The problem with injury after 2018 is the $17 million franchise tag will not count towards the contract guarantees and the Cowboys will have to wait longer to get out of the big contract. You also have the risk you're paying out more if Lawrence performs as well or better than he did in 2017, which we hope he will.
If by chance Lawrence does fall off the map next year and Dallas was saved by using the franchise tag, then we're still faced with either not having a good pass rusher or by overpaying a dreaded free agent to replace him, which the same risks would then apply to the new free agent.
Sign the dude to a long term contract already. The risk will never go away.