That is a pretty good deal. Japan only has two carriers and getting a SIM would have been more expensive, and give you much more limited data. A 2GB SIM is almost $40.
It's odd that Verizon wouldn't turn on Travel Pass because you are prepaid. I looked on Verizon's website and it says nothing about prepaid customers in Travel Pass.
Since I am on an unlimited voice/text/data plan. This was more economical for me. I also don't have to worry about my phone# changing. Also, if I decide to turn off my mobile data for a couple days, I can still make WiFi calls if I am in a WiFi hot spot, which are all over the place here. Verizon doesn't allow WiFi calling (other than Skype, Google Voice, etc) on non-Verizon branded (i.e. unlocked) S8/S8+ phones.
Also, Travel Pass in Japan is fairly recent, like in the last three months recent. I was here in Okinawa back in Feb and Travel Pass was not an option.