"if suddenly we've got useful works, such that you can apply that $X million in electricity to solve problems that are actually not wasteful, and worth perhaps $X million dollars in solutions"

first if we ignore the part "not everyone is going to have equal access to people who are willing to pay for solutions of useful work"

I think this can never happen.

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People should think PoW as lottery.

Every X minutes there is a prize draw giving you Y coins.

But ticket price converges to market value of:

ticket price = (Y coins)/(total tickets sold)

So if you add a value to 'not winning ticket' by introducing 'useful work', you are changing ticket price to:

ticket price = (Y coins)/(total tickets sold)-'work value'

so you are just giving coins more value, means making 'new ticket' more expensive.

new ticket = old ticket + work value

In the end you will still 'waste' the same amount at equilibrium.