Capitalism is literally defined by the ability to invest capital to accumulate more of it by way of profit. The logical end of this process is straightforwardly monopoly.
And if the world/environment/context of the business didn’t change then the monopolies might last, but because there is change there is room to innovate and outcompete the monopolies.
Capitalism is not only defined by the accumulation of capital, but also by competition. The interplay between market forces, competition, innovation, and regulation in capitalism works against the formation of monopolies.
The aerospace industry is not a good example of capitalism. What we have with Boeing is basically a government sanctioned monopoly. It’s basically a weak form of nationalization, without the stigma.
>Capitalism is not only defined by the accumulation of capital, but also by competition
That's wrong. In reality, the mere theoretical potential for competition has always been more than enough to call it capitalism from any perspective. The facts are that actual competition is not a requirement.
Capitalism is simple: The capital rules supreme. As opposed to the previous system of aristocracy, where it was the land owners. Nobody would seriously claim that aristocracy requires any kind of competition between the aristocrats. Even the very first capitalist big enterprises, such as East India Company were created as _monopolies_