A lot of us pinball fans are collectors at heart and still hold onto the past (Heck, we still love pinball). I am very happy to have found a retail copy of TPA for PS4, since it had sold out on Amazon so fast. 10-20 years from now I would still like to be able to play this game on PS4, and physical discs are the only way to ensure this.
If there are currently no plans for a physical release of future seasons, hopefully you can consider publishing them in small numbers and selling them directly through your site? I know that I myself would pay a hefty premium for limited edition physical releases.