If that we're true you'd have a point, but the good news is that isn't.

It's possible using cryptography to construct proof for statements like "0xDEADBEEF is the hash of the tip of a blockchain starting at the genesis block where all rules pass, with total difficulty Y", where the proof is much smaller than the blockchain (in some cases only a few hundred bytes).

Such systems are already in production use for small programs today. Scaling them up to work over the whole bitcoin blockchain is a (considerable) engineering exercise, but I think it's inevitable.

(And you should hope it is, because we've already seen what people do when validating the history becomes too expensive-- they skip it)