In a defence of Pro Tec they design their covers for specific models. This makes them a very snug fit on the actual van for which they are designed. They do not give a general size for any number of vans. For them to try and recut and sew a cover to fit a totally different van is probably as expensive, if not more so, than a new cover.
I feel it is being unreasonable for people to expect them to accommodate the various changes of minds that may occur after they have made and sold a cover. If people decide to change a van, then that is their choice.
There are similar issue with awnings. I have 2 in my loft from previous vans but I would not expect the supplier to cut and make the awning to fit the new van, without a lot of cost.