I don't think you can remove a discussion about changes in allele frequencies from environments. Thus, making 'other things being equal' is not a reasonable premise from which to discuss evolution.

I do see the argument you are espousing but I would only suggest that you consider the broader context of what happens when new species arise. I suspect you care more about the elimination of alleles (which is surprisingly hard to do), but this is a good example for why I disagree:

One possibility for speciation is the creation of a new niche (lets say a new type of tree grows on an island, its seeds having been carried there by an unusual current). Here, organisms might have increased reproductive opportunities over their neighbors by exploiting this new tree. In time, one could imagine organisms that live in the new tree, like microbes, could acquire enough new features to constitute a new species.

Consider the case where organisms from the original population become unable to grow on the new tree. If the new species is also unable to grow in the original environment, how can we say either is more or less resilient?