Quite often you can fly above an embedded line of thunderstorms when above FL 410 but right in them at FL280.
Also extra fuel usage means less reserves- wait for a repeat of Norfolk Island accident where the aircraft was held low because it could not get a RVSM dispensation.
I don't blame the ATCs for this- they would like to be able to handle the small number of non ADSB compliant aircraft in the safest way but are not allowed to.
But wait and see who will be given part of the responsibility when an accident occurs. Suggest you look up the Navair mid air at Bankstown many decades ago.