The plague is somewhat like a forest fire, in that it does a lot of damage but passes quickly. Some trees are better at recovering after forest fires (thicker bark, ability to shoot from below ground, etc.), and the same applies to defoliating plagues, like yours. I expect most of the trees should recover from this year's invasion. By the time new leaves appear, the caterpillars should have moved on or should be past the larval stage, so the new leaves should survive and start to rebuild the trees' reserves of starch. If the plague returns next year, however, that may be more of a problem, as the trees' reserves could get depleted.
Caterpillar plagues can also be a problem to local ecosystems (as well as to the trees themselves), in that the literally hundreds of other species that feed on oak (eg.) will be in trouble; and the birds that feed their young on those species.
But such plagues can also be self-limiting. Once the resources are used-up or disease takes hold, the exploding population crashes, and the area becomes repopulated from adjacent areas. Regaining the previous balance can take a while, however, as some pioneers can be better exploiters than others; they can dominate the race to grab the old niches. While the old order is re-establishing itself, temporary niches and opportunities occur. For instance, light reaching the forest floor in summer may permit some species of plant which normally flower at different times, to flower at the same time thereby allowing some hybrids to arise. Patient understorey trees like holly and yew get a chance to put on an extra spurt of growth (normally deciduous trees around them grow, mature and die while they slowly make headway using the lower total annual light budget arising from the deciduous canopy above them). Speckled wood butterflies will wander the sunny temporary glades and may reach newly colonizable sites. Holly blues may take advantage of the fresh holly growth. Thistles may fill what would normally have been a dim forest floor, providing a temporary feast for fritillaries (which may lay their eggs on or near burgeoning violets). And so on.
On a wider scale in time and in space, plagues are part of the normal functioning of the ecosystem. It can be hard to believe that when you're in the middle of it, I know.