They also shouldn't trade their starting goalie till they know they have a better one lined up.
Waiting till the middle of next season won't kill his trade value. Dubnyk returned a 3rd as a free agent last January despite only been one year removed from a .891 season.
Maybe they could get 2nd if they trade him this year. It's stand to reason they would get more with the extra year on his deal.
As far as next year, you can always find average goalies on the market for cheap. Maybe the Flyers luck out a get another Neuvirth like (so far) performance for a year out of the guy they sign. Even if they guess wrong, it doesn't really matter. The Flyers with Mason won't be challenging for the cup next year anyway.
So the question needs to be answered is do the Flyers view Mason as the goalie for 2018/2019 and thus willing to commit to him before his contract runs out at or greater than his current cap hit for term. If the answer is no, trade him when you can get the most for him. If they think they can get him for cheaper once he hits UFA and sees the market, then get the asset and let him perform for some other team one last year and then try to sign him for cheap. The only risk is a bad season next year. What's the reward if they keep him next season, a first round exit?