"The most significant fact is that it will not fall from inside pressure. Only outside, foreign, hostile intervention can destroy it."
In fact, some historians have argued the exact opposite: the Soviet Union only lasted as long as it did because the Kremlin held up a foreign enemy to distract from internal pressures. Without a boogeyman to act as a unifying force papering over domestic cracks, the Soviet Union would have imploded long before 1991. As Dr. Wade Huntley puts it:
According to this view, had it not been for a tendency toward wild-eyed anti-communism on the American side, the Soviet Union may have collapsed under its own weight much sooner than it did. The stridence and belligerency emanating from Washington, from the 1950 adoption of "NSC-68" onward, had little effect but to strengthen comparable hard-line views in the Kremlin.
Armstrong, David, and Erik Goldstein, eds. The End of the Cold War. Routledge, 2013.
This theory is not the most popular with the public, but it enjoys respectable support among experts. None other than Truman's ambassador to the USSR George Kennan, who was once a leading advocate of containment, argued after the fall that:
What did the greatest damage was not our military preparations themselves, some of which (not all) were prudent and justifiable. It was rather the unnecessarily belligerent and threatening tone in which many of them were publicly carried forward. For this, both Democrats and Republicans have a share of the blame.
Kennan, George F. "The GOP Won the Cold War? Ridiculous." New York Times 28 (1992): A15.
In the same article, Kenan further advanced another theory, namely that the actions or policies of the West - chiefly the United States - were actually not very important. He implicitly affirms the idea that the Soviet Union fell to domestic factors, dismissing the idea that an external power could cause such domestic upheaval as "childish". In this view, the Western allies were little more than bystanders, witnessing the fall of the Soviet Union to internal pressures.
Louise Bryant's predictions, while certainly idealistic, were also an understandable product of her circumstances.
Vladimir Lenin, the founder of the Soviet state, once said that the struggle between socialism and capitalism would ultimately be decided by the productivity each side was able to achieve, not on the battlefield. And he was right in essence, if not wrong in picking the winning side.
Pechatnov, Vladimir. "Soviet-American Relations Through the Cold War." The Oxford Handbook of the Cold War. Immerman, Richard H., and Petra Goedde, eds. Oxford University Press, 2013.
When Byrant wrote her Six Red Months in Russia in 1918, it was not unreasonable to make the same mistake as Lenin. Communism was still essentially theoretical; the idealistic belief that it could compete favourably against capitalism in terms of economic output had not yet been undermined by decades of underwhelming reality.
Meanwhile, belief in the inevitability of class war underpinned the ideology. In fact, shortly before her book was published, the Allies did intervene against the Communist revolutionaries. Had their attempt to prop up the White Army worked, Byrant would've been completely correct.
What this really shows, is that predictions of the future rarely works out.
Of course, as you pointed out, there's still considerable debate over the exact causes of the Soviet collapse. Which make sense- the Roman Empire fell in the West over 1500 years ago and entire careers are still being made debating why it happened.
However, it's important to note that even those who attribute credit to American administrations, generally do not dispute the fact that the Soviet collapsed under domestic tension. No one contends that a foreign invasion ended the Communist government, as Louise Bryant thought would happen.
Instead, the primarily debate is whether the Soviet collapse was inevitable, and to what extent the Western Allies added to that pressure.
In the end, George Kennan's view that the Soviet Union would collapse under its own weight was accurate. Whether U.S. actions increased that weight or whether collapse was inevitable on its own schedule remains unknown.
Watson, Cynthia Ann. U.S. National Security: A Reference Handbook. Abc-clio, 2002.