Of course not. But, you wouldn't run out of the energy to cast spells for some time.

[Of course it knows that.]

Their energy is what helps to stave off heat death, but it's more than that, of course.

In our home universe, and from what we've seen in other realities where we've had contracts, magical girls are integral to the evolution of the species. Their magic and their legacies have had a very profound effect on bringing their civilizations to advancements that wouldn't have otherwise been possible.

Quite a few places would lack even the basic understanding of how to make fire without the existence of magical girls.