Investing in BTC as a collectible unit is a huge speculation, I have no doubt about it. No matter how other Bitcoin supporters talk about "such a useful ledger", there's no way to deduce how much all BTC units should be worth. Bitcoin-as-network is as good at $10/BTC as at $1000000/BTC. You still can cheaply move dollars across the globe, or timestamp your contracts, or whatever.

The productive discussion would be about what is the basis of that speculation, or whether it can be equally huge with any other asset like a tulip bulb or dotcom stock.

I'd argue, Bitcoin is unique in a sense that it can (already does) easily act as money itself. People were investing in tulip bulbs, ponzi and dotcom stocks in order to cash out big way. Current stock market is no different. There's a lot of "bad" money pumped by Fed and from pension programs and no theoretical sustainability while all fundamentals say it's going to crash hard.

On contrary, Bitcoin is a risky long-run bet that it will be useful as a world money, so there would not be any need to "cash out" into some single asset at the "maximum" price. If Bitcoin succeeds, then there'd be no need for dollars, euros, yen, gold and silver. You'd be calmly selling portions of your BTC to buy food, travel, shelter and invest in business.

Some portion of investors bets on that exact outcome. Those who bet on short-term price increases to "cash out" produce that pyramid-looking effect (huge bubble and then huge price drop), but every time there are more and more long-term investors. This time BTC went from $140 to $1200 and quickly crashed to $500 (then, getting back to $800-900). Clearly, everyone who wanted to cash out already did and what's left are those holder who are not selling for some time now. When BTC goes to $5-10K this year, then they might sell a portion of holdings to newcomers, but keep the rest for a really long time. If everyone continues to follow the same logic, this "pyramid" will lead to a situation when almost everyone has some BTC, everyone values BTC a lot and everyone is ready to trade this BTC between each other for whatever goods and services they desire.

Right now, most investors in Bitcoin are economically disconnected (do not trade directly with each other), so you don't see much transactions. Bitcoin is used like gold: a speculative "store of wealth". But at some point the density of investors will become so big, that more and more direct economic relations will appear between them. Tomorrow you might find that your local baker is BTC investor and you can pay him in BTC. Later he will discover that his supplier also invested in BTC and thus will be able to accept even more BTC and pay his bills directly with BTC. As this second phase (increased trade bypassing fiat currency) progresses, people will see how Bitcoin is no longer just "gold 2.0", but actual money. More and more will invest in it and thus become payers and payees in BTC. As this goes on, reservation demand for USD will start to go down quite noticeably and people will rush to BTC even faster (as prices in USD grow quicker and quicker). Ultimately, USD will hyperinflate and become worthless. All tax collectors will go looking for another job (they want to be paid in BTC, but government can't just print it or buy it). In the end, inflation and taxation will become impossible. Police will have to be sponsored voluntarily on district-by-district basis. Armies will abandon their bases and poor islamic nations being raped for decades will be paid a handsome retribution as an anti-terrorist protection measure by the wealthy Bitcoin holders in U.S. Bitcoiners outside the U.S. and U.K. would not need to worry about being blown up that much, yet still will contribute greatly to protection of whatever cities/countries they reside in.