In fact, it is precisely the reverse of that: it's not a buyer of last resort, it's an energy price FLOOR. Any energy that you could sell to a customer, must be sold above "y". And so it is with computer hardware - any top or near top wafer capacity item you may want to buy must be above "Y" (what a crypto miner would pay for it). And this is why we saw massive price hikes for consumer computer tech in the last two years.
And is this way - a price floor - and not the way you describe it, because of the economic incentives of miners. They have already paid for these captial intensive mining rigs, and to best turn a profit they must be running at all times. The marginal cost of mining is important, but given the capital costs (incl depreciation of hardware!) you cannot ignore it.
Basically, your explaination is a failure of first order thinking. To a first order approximation, only the marignal cost of mining matters and thus the scenario you describe is true. However, you must include the second and nth order effects of capex to truly match reality.