I have changed cars due to MPG reasons with motability on the xc60 so it can be done. The vehicle is meant to help keep you mobile if it doesn’t it sort of defeats the objective. Im my case I could afford the costs but it was ridiculous and this was explained to motability and a change was agreed without have to pay the fee.
I’m pleased for you, but surprised. If a few people did what you did it might go some way to explaining why cars like that are no longer on the scheme and why we are left with mostly small / underpowered SUV’s – but at least the mpg is good ?