There's no technical barrier to truncating a blockchain. Just create a new genesis block and have it initialize every account balance to the balance it held at the end of the previous block chain. You could discard all the junk data as simply as that.
"No technical barrier" means nothing with respect to Bitcoin. You have to convince a large majority of Bitcoin users / miners / exchanges / services to switch to your new genesis block or other protocol modifications, otherwise your fork will die off, or at least not "be Bitcoin" as the rest of the world sees it (e.x. Bitcoin Cash, etc)
But really it's not even necessary, nodes can and do prune op_return, spent, and otherwise unspendable transaction outputs.
The point though is someone is likely to always hold onto a complete history of the Bitcoin blockchain, even if most users don't.
