Bitcoin transactions are irreversible.
This is false.
True statements would be:
Well confirmed bitcoin transactions are prohibitively expensive to reverse. Unconfirmed transactions in some situations can be trivially easy to eliminate
This is six of one and half a dozen of the other. I can just as well say that someone could buy all bitcoins out there, already mined or to be mined, and then reverse any transaction he may feel inclined to. And I assume he might not actually need to buy
all bitcoins...
Just enough to make mining no longer relevant