Wouldn't a government be easily capable of controlling the currency by having the 51% of the computing capacity? Also, by deciding on ana accepted fork of a given crypto-currency, and deciding on technical aspects of how does it operate (e.g., time related), a government could have a great indirect control over it.

In general I find this article clear in it's belief that that gov't will staunch Bitcoin/Crypto. I don't see any evidence of the U.S. headed there... yet. Rogoff offers only precedent, which is not-nothing.
One key detail about why crypto is so attractive, and I think may reveal a lack of basic understanding by Rogoff on how crypto works is this quote, "Even though Bitcoin transactions require an exorbitant amount of electricity, with some improvements, Bitcoin might still beat the 2% fees the big banks charge on credit and debit cards."
That's patently wrong.
Bitcoin transactions require as much electricity and cost as much as it does for me to post this comment: basically none. MINING bitcoin, a wholly different operation that only happens one time per coin does consume a ton of electricity. That coin can then be transacted limitless times at ZERO cost.

Ken Rogoff is entirely right on the long term fate of bitcoin, but like many observers places too much value on the ersatz blockchain technology. The only real use for blockchain is bitcoin. It is an entirely inefficient technology by design that really is just a form of encryption. There is no magic here. The vast amounts of time and money wasted on blockchain will rival the sad misallocation of resources on bitcoin.

It is a massive bubble which could go higher but like the Great Tulip bubble it will crash and wipe out. Also, I question the sound backing of the Bitcoin and other Crypto currencies, not backed by anything of value such as Gold.

Many people have seen their Crypto Currencies disappear or accounts frozen by hacks or exchanges closing down.

As respects to Ken Rogoff and his push for Cashless society, forget it Ken, not going too happen for so many reasons.

If you can tell me how I can take my Bitcoins from the cyber universe where they will exist, and buy a house in this physical world where I exist, you will have sold me on to the idea of this alleged currency.

I recognize the concern regarding Bitcoin, based primarily on its early developers who looked at it as a way to breach rules, aid illegal trading and clock a snook at the authorities. It's a shame, but it wont be the last time this sort of thing happens - the development of ecommerce in the 80's and 90's was quite similar.
But we got through that; it took a sharp pull-back to achieve it but we now have a multi-trillion dollar market working too well. The concern now is that its efficiency is wrecking the traditional shopping pattern at malls an corner shops, through providing better, faster, cheaper service with well-trained operatives standing by to correct any weakness in the chain.
Blockchain is showing how it can do the same for banking (actually for much more than banking; really in all matters of finance, but let's concentrate on just the payments space for now).
The ability is there for international transfers, such as family remittances, to go through in real time at a fraction of current costs, i.e. down below the SDG 2030 target of 3% from today's 7,4% average, with full control on KYC, AML and all connected data for law enforcement - so, the most compliant and secure format to date, beating out the legacy systems of SWIFT and the banks which have increasing problems with breaches of data and loss of large sums of money.
But here is what every economist, every government official, every regulator and, yes, every citizen of the most needy countries in the world can want to see; this can all be done in a 'bottom up monetary policy' BUMP for short, where the concentration is on helping those with the greatest need.
Don't let the Bitcoin become the tool of the investment banker and the money launderer, wheeling hundreds of millions of bitcoins in and out of the market, causing honest small investors to be stopped out - let the trillion dollars of remittances follow the tight rules on size, maximum $900 per month, no grouping, under five node authorization approval (usually in less than a second) - become the main market.
Let us learn from the BUMP - but let the BUMP do its job of lowering costs for the poor, maybe giving them a way to save some money as well - in something that does go up in value significantly - and allows hope of finding a way to break out of poverty.
Let us see what can be done; let the poor lead the way - and actually start the narrowing of the rich/ poor gap with this exciting technology!
What can you do to help?

Kenneth Rogoff is fighting for the embarrassing position of becoming the new Professor Bitcorn. He's are clearly baffled as to why millenials have massive interest in bitcoin, and mate, its not because we're all hanging out for the Central Bank Coin rip off of it to come online.

I'll take the first opportunity to comment and would like to thank Mr. Rogoff for going on record with his views of Bitcoin. In the very near future, I think we can look back on Mr. Rogoff's analysis and conclusions about Bitcoin to see his position has major omissions and errors of fact. Of course, one would not be surprised by this one bit, given Mr. Rogoff's recent history of data omission and lack of basic fact checking on a major piece of academic research.
(See https://www.theatlantic.com/business/archive/2013/04/forget-excel-this-was-reinhart-and-rogoffs-biggest-mistake/275088/)

It's fairly obvious that Mr. Rogoff looks only for information and conclusions that support his hypotheses, rather than considering the evidence that refutes his claims. What good is research if this is the author's end goal. Besides, economists as a whole fail miserably at predicting any meaningful information, even in the very short term! How many economists predicted recessions in countries around the world even 6 months before the global collapse? The Answer: ZERO.

But for now, we can let Mr. Rogoff use his credentials as a PhD to make predictions about Bitcoin and other digital currencies. However, when the future arrives there's going to be no doubt about the failed accuracy of his conclusion.