Who is paying for the running costs of Bitcoin? Bitcoin mining will consume around 70TWh for 2020 (~3 nuclear power plants).
As far as I know, electricity is paid using fiat money. If 1 kWh costs 0.1 $, without counting hardware costs, that means that 1 year of mining costs $7 Billion for electricity.
At the moment, the Bitcoin capitalization is around 357 B$.
It follows that 2% (7/357*100) of the capitalization is used by electricity every year!
Who is paying for that (and how can it be sustainable)?

people who invest in Bitcoin using fiat money?
by Bitcoin deflation comparatively to fiat money?
people who lose money when/if Bitcoin goes down?
more generally: Mafia (an expensive laundry is not a problem if it is good)?
or I don't understand enough how Bitcoin works, and that's something else :-) ?
The people operating the mining hardware do. Mining hardware consumes electricity, so people who run the hardware receive an electricity bill from their electricity provider. They pay the bill using the proceeds from mining. Usually miners will sell some portion of the Bitcoin they earn for fiat, and pay for electricity using that fiat.