How would that actually help long term though? Imagine they write off their entire entire fleet in 1 year. Yes they pay less tax that year, but in future years when they sell their old cars that are worth nothing for lots of money they'll have a load of extra tax to pay.

I can see how it lets you move tax around in time a bit but that's it (and it's often allowed explicitly anyway).

Am I missing something or is this the common "tax write-off = magical money" misunderstanding?