I don’t think there is a crystal ball, that can give you the answer.
At the moment Motability has very few diesels on the scheme as your aware of. Whether that is because of the current situation with the markets or Mb themselves are decreasing the number of diesels on the scheme in favour of hybrids.
Diesel’s and petrol (ICE) will be sold untill the ban comes in place in 2030, so would be available in the retail market, however they are slowly falling out of fasion, with EV’snow outselling diesels.
As already pointed The increased price of diesel has narrowed the gap between running a petrol, my last vehicle on the scheme was a diesel and was just a few pence cheaper to run thy my current vehicle when I worked it out at the current prices. I doub’t we will see diesel at prices they once were.
My advice if want diesel jump now and get the Tiguan, but don’t discount Hybrids.