The minting should be seen as digging for gold in a gold mine. This is similarly wasting energy and resources for actually not doing anyone any favor since you increase the supply of gold. Except for yourself of course, and that why people will always will dig for gold as long as the price of gold dug will be higher than the cost of resources needed to dig. However, this difficulty is what gives scarcity and therefore value to gold.

Similarly, if and when bitcoin becomes more mainstream and follows economic sense (rather than beeing driven by curiosity, fun and excitement for participating in something new) the cost of electricity (in dollars or equivalent bitcoins) to create a bitcoin will never exceed the value of the created bitcoin. If that would happen, many will shut down their bitcoin generation for it would make no economic sense (I already stopped mine!). Luckily, and unlike gold mining, this behavior will adjust the cost of creating bitcoin to a sensible level (below the value of the generated bitcoin)

In the long run i expect that bitcoin minting will indeed be left to the supernodes which will probably have specialized hardware  that are excellent in hash calculations and can do that way better than anyone with a general purpose pc. Similarly to specialized gold digging machines.

So although you are right that it would be nice to find a system that provides scarcity without cost (I dont know whether this is possible, it seems not), the costs are certainly connected and less to the value of the created bitcoin. And it is a self regulated system that will increase the cost of creation only if the use of bitcoin actually adds value (in the form of ease of use, anonimity etc.) and therefore is worth it.
The cost of creation is therefore is no limitation of the growth but more and indication of the added value of the use of bitcoin.

fuuny note: Following this logic the cost of minting will be somewhat less than 21million bitcoins which with the current prize of bitcoin is  around 1 million dollars.

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