The economists claim that Bitcoin is a bubble basically for one reason: the liquidity premium of (or, in other words, transactional demand for) bitcoin is huge relative to its intrinsic value (IOW, the value you get when the music stops).
Since bitcoin is actually the first asset to fully conform to quantity theory of money, most economists are perplexed.
Could you expand more on this?
What do you mean by liquidity premium (transactional demand)? If you mean transactional utility, then it is pretty slim in Bitcoin since Bitcoin is not used as money. It is a vehicle for speculation (the proverbial "buy low, sell high") but once you inject your bitcoins (or other cryptos, for that matter) into your exchange account, they mostly stay there, i.e. you don't transact with Bitcoin as much as you use it for speculation. So it is really minute in comparison with demand for Bitcoin as a speculative asset. Also, I'm not quite sure what you mean by Bitcoin's intrinsic value. As far as I can say, there's none
I agree that bitcoin does not have any important intrinsic value. By this I mean that it is not a security entitling the holder to anything and that its objective, market-independent properties are almost useless (cryptographic timestamping on [Suspicious link removed]es to my mind as one). Bitcoin's value comes almost solely from transactional demand for bitcoin and expectations of future transactional demand. It is not the same as demand as a payment service for coffee or t-shirts, but rather demand for a liquid asset, which is easily transferable globally and which is able to settle even large transactions swiftly, reliably and pseudonymously.
While it is true that speculation is the main driver of bitcoin volume, but those who believe bitcoin prices will rise can do so based only of future transactional demand (or are irrational but lucky). Every time one does the calculations like "imagine bitcoin will be the size of VISA" or "if 1% of global capital markets' funds were directed to crypto..." one is estimating the future liquidity of the asset. Anytime short term trader looks on the charts in the search of the technical patterns he tries to estimate money flows. And in fact speculation very positively influences bitcoin liquidity and the premium people will pay for such liquidity. So it is a positive feedback loop.
Sure, bitcoin is not money, since it is not a unit of account anywhere, but it is a (small, nascent) medium of exchange. And since economists are accustomed to seeing large liquidity premia on monetary assets and not on (virtual) commodities, they shout "bubble"!