The bus transportation is slow as it is. Extra time would be needed as the bus driver checks each passenger's card. SSR has five or six stops, too.
Besides that, according to Disney's own information it is not required that you be a resort guest to use the buses, just that you have a park ticket.
I also don't see how paid parking at DTD impedes commerce if validation is available. Commerce at DTD is more impeded by those who use the DTD lots as free parking for the theme parks. Of course it would only stop those who use cars.
Checking the cards would be easy enough, more time yes, but total stop time shouldn't be affected severely (plus there will be less people to check

Validation works but it is self limiting. People will stay for as long as it is free. I have a local mall with parking fees that went bankrupt. Many of the shops have closed but the movie theater (that validates) has remained viable. You would think that the restaurants would still do well (Dinner and a Movie...) but the theater only validates enough time sufficiently to see a movie. Most come to see the movie and leave shortly afterward without sitting for dinner. In DtD's case, How much time is enough? 3 hours may not work for Food and AMC. 4 hours might be just enough for AP holders to do rope drop, several rides and come back. The scammers will go to the parks, come back for lunch, cycle the car through the gates, and return to the parks.
As an SSR lover, I've encountered the full buses and the packed parking lot with few patrons at the shops. I want DtD to prosper but I do not want parking restricted. Available parking is important for commerce. Free parking is even better. $14 parking will not work for DtD and validating has some issues. Also, I do not want to pay for more buses because "they are using them".
If Disney is losing revenues, this will be addressed. If it's just an inconvenience to us, it may never change.