[inimino]

There seems to be a mixup here between "pressure" as in "this organism is under pressure" (implying an unfavorable environment) and selection pressure, which acts on genes, not species, and is really more of a filter.

It's not as if an organism can "release" the "pressure" by evolving in a new direction. In your example, if we eradicate mosquitos, one transmission vector becoming less viable doesn't make other vectors more likely to arise, as if by some conservation of total population.

> least costly alternative in response to imposed pressures

Evolution is even lazier, alternatives don't arise in response to imposed pressures at all, so in this scenario the lazy thing is extinction.