big nail : flat tyre

[Roadster]

I went to the tyre shop this morning to drop of the tyre.
They told me they are not allowed (car is a lease) to fix the puncture with the usual mushroom as the tyre is a runflat...
Reason is they can't see if the steel structure in the tyre is damaged or not... told them I pulled over immediately after hitting the nail... but they don't want to hear it. The same thing happened to me on my E61, had the tyre plugged with a mushroom, the tyre blew up about 3 months later, so maybe they're not wrong... Anyways, by Monday I should have a new tyre installed.
not my problem because I have extra tyres included in my lease, but it feels like a waste to throw away a tyre that could have lasted for another 10k kms...