"Bitcoin is too inefficient to be a currency."
What is the author's level of expertise on Bitcoin? Bitcoin is less efficient in terms of computing resources (which are cheap and getting cheaper all the time) It is more efficient in terms of being able to make irreversible, global payments in any amount. Bitocin is not a static entity - it has improved massively over the years and continues to improve in terms of efficiency, scalability, and security.
It would be nice if Bitcoin critics would give scenarios in which they would admit they were wrong about Bitcoin. Otherwise they can continue to move the goalposts as Bitcoin continues to grow and improve.

"Thus the sole way most promoters will realize value from their bitcoin holdings is through new entrants into the market."
Actually, one of the reasons that bitcoin is so undervalued is because many people (such as the author of the article above) don't yet understand the value. Bitcoin has network effects - like Uber or Facebook, it becomes more valuable as more people use it. Pyramid schemes are a specific thing where payments are made up through a hierarchy. Bitcoin does not have a hierarchy. Bitcoin is not centralized - it is decentralized.

"The “store of value” argument also depends on new buyers coming in to support those who want to liquidate their holdings."
This isn't true. Existing holders of bitcoin may want more. Long term holders of bitcoin do not want to liquidate their investment. they may want to sell a portion, but the idea is that they have a new form of money. They don't need to go back to an old form in order to buy goods and services.

"Again, this suggests a pyramid scheme, albeit one that doesn’t promise explicitly high returns."
Bitcoin itself doesn't promise high returns, or even a profit. This is because it isn't a pyramid scheme.

"Whatever it is, it is not a legitimate investment." The author admits he doesn't know what Bitcoin is. Why does he think it isn't a legitimate investment?

"With a few notable exceptions, governments stabilize their currencies and don’t sell them to the general public as speculative investments. For these exceptions, more stable currencies are available."
In those places, Bitcoin may be available while stable foreign currencies are not available. In those cases, bitcoin outperforms the local currency consistently, while having other desirable characteristics (suitable for fleeing the country even when destination is unknown).

"But gold has other uses: To fashion jewelry and other art, for example, or even as doomsday currency should electricity and internet become unavailable. Bitcoin can’t serve either of these roles."
Gold is used a hedge against inflation. Bitcoin is used as a hedge against inflation. Why did the author leave this out?

"But other than for speculation, bitcoin has no utility beyond such activities."
Bitcoin has many other uses. When people all over the world use Bitcoin, they don't feel the need to notify the author. The author must actually do some research and not merely assume his conclusions.

"Whatever they have to offer, one does not need to purchase cryptocurrency to use blockchain."
This is literally what owning bitcoin gives you the right to do - it gives you the right to write to the blockchain. If you don't have any bitcoins, you cannot write to the Bitcoin blockchain. If you make a centralized blockchain, it has no reason to exist.

"How is bitcoin different from other pyramid schemes, say, those run in penny-stock boiler rooms?"
Bitcoin isn't a penny stock, and it isn't sold via boiler rooms. That was an easy question.

"Some no doubt truly believe that bitcoin will function as a currency for enabling transactions, rather than a “store of value.” But I wonder whether such people truly understand economics, our monetary system, or our business environment"
I understand economics, our monetary system, our business environment. I also understand Bitcoin and I know that it already functions as a currency for transactions.

"These critiques are not unique to bitcoin, but apply to all cryptocurrencies."
Some cryptocurrencies are similar to Bitcoin, while others are wildly different in terms of monetary policy, governance, and structure. The author doesn't understand Bitcoin - and now he is making statements about all cryptocurrencies?

"Because the fact is, if it looks like a pyramid scheme and sounds like a pyramid scheme, we should treat it like a pyramid scheme until proven otherwise."
It has been proven otherwise. Bitcoin has no one running it, it doesn't have a hierarchy or mandatory payments, it doesn't promise profits.

The author could start with my article here:
https://medium.com/@d.ameli/13-common-misconceptions-about-bitcoin-45e1…