As a corollary to this, the cost per transaction also varies linearly with the price of BTC - the more useful BTC becomes for investors/speculators/as a store of value, the less useful it becomes as a tool for transactions.

On the other hand, the high valuation can only be justified by the expectation of it being a really good tool for transactions (or, alternatively, the greater fool theory) and becoming bad at transactions means it'll eventually become also a bad store of value; so a technical solution to transaction cost is highly needed.