Monopolies don't need to be eternal to be a monopoly. They will fail when they make a mistake or become obsolete. What they won't do is lose to a competitor on their own terms.
For instance, IBM is a recognized mainframe monopoly. They are not valuable today not because they lost the mainframe dominance, but because mainframes are obsolete (or almost).
Microsoft outmanuevered IBM multiple times. IBM tried to dominate the PC business, was successful for a few years, and then failed.
> What they won't do is lose to a competitor on their own terms.
Markets are always changing. Large companies tend to optimize for a particular market, and when it inevitably changes, they cannot adapt and get left behind. Often its their competitors that change the market.
See the classic book "The Innovators' Dilemma".
(The same thing happens in nature when a species over-specializes to a particular niche.)
For instance, IBM is a recognized mainframe monopoly. They are not valuable today not because they lost the mainframe dominance, but because mainframes are obsolete (or almost).