I don't think "what about the people who have paid for parking and coaches" is a particularly valid one.
This is next season we're talking about, not this one. As far as I'm aware no one has paid for parking tickets or coaches for next season, so are not being charged. Come July they will be buying season tickets that include the park and ride. They then have the choice to pay extra for parking or coaches if they wish, but they will be doing so knowing they already have P&R vouchers. It will be their choice to pay extra for alternate arrangements to those included in the season ticket.
It's not like the club are forcing people to pay extra this year for vouchers when they have already sorted their travel arrangements.
But the club will still need (approximate figures):
6,000 people to park on site;
4,000 people to walk or cycle;
...and there will be around 4,000 OAPs who don't have to pay for travel.
However you look at it around half of the crowd should not have to pay for travel.