That doesn't mean that we should assume modern primitives and constructions are unbreakable (though the trend has been for protocol designers to seriously overestimate the likelihood of _any_ primitive being broken).

What it means is that instead of designing and implementing elaborate handshake mechanisms, which inevitably succumb to downgrade attack bugs or, often, worse things, we should version entire protocols.

So obviously, if the ChaPoly stack is broken at some point, people will need to update WireGuard (just like they update OpenVPN when TLS bugs are discovered). But they protocol doesn't need to account for anything more than the fact that it might someday be replaced with a different protocol using different primitives.

The "agility" protocol designers are looking for has probably, historically, been implemented at the wrong layer. As usual, the closer something is to policy or prognostication, the further up the stack it should go. TLS bakes this stuff into the transport layer, which has hamstrung us and, ironically, not actually done much to protect us against vulnerabilities. WireGuard makes a different, better decision.

There's no tradeoff here; this decision is by itself a reason to adopt WireGuard.