Your belief that "power structures can't be fixed" perfectly illustrates what educator Paulo Freire described as the oppressed having a "diffuse, magical belief in the invulnerability and power of the oppressor". Anthropologist David Graeber noted that modern capitalism has constructed a vast bureaucratic apparatus "designed, first and foremost, to destroy any sense of possible alternative futures" and to ensure that challenging existing power arrangements seems like an "idle fantasy". The idea that the platform holder holds all the cards is an ideological tool used to encourage passivity and convince you that your only option is to submit.
As James C. Scott demonstrates in his analysis of authoritarian systems, any formally organized, rigidly planned system is ultimately parasitic on the informal, unscripted practices (which he calls mētis) of the people within it. A closed system cannot survive on its own rigid rules; it requires the constant, active cooperation and practical know-how of its subjects to function.
Gene Sharp's foundational theory of power echoes this: no regime, corporation, or totalitarian system possesses inherent power. Their power derives entirely from the cooperation, obedience, and skills of the people they govern or employ. If blue-collar technologists, developers, and users collectively withdraw their skills, labor, and cooperation, even the most monolithic tech empire can be paralyzed. The power of the OEM is not absolute; it is entirely contingent on your continued participation.
You point to the GNU/FOSS movements as successful because they ignore corporate nemesis-building and instead focus their volunteer hours on creating "something wonderful."
In the study of nonviolent struggle, building alternative social institutions and alternative communication systems are indeed recognized and highly effective methods of intervention. Furthermore, creating "commons" (like open-source software) is crucial because it provides a practical model for a non-commercial way of life.
However, building alternative commons is not a substitute for directly challenging power. As Silvia Federici argues, creating commons must be seen as a complement to the struggle against capital, not an alternative to it. If you only build wonderful alternatives without contesting the power of private capital, your creations remain vulnerable to being enclosed, commodified, or crushed by the very monopolies you are trying to ignore.
Ignoring the oppressor does not make them disappear. If technologists want to reclaim power, the first step is to reject the neoliberal fatalism that views the current corporate dominance as an unchangeable law of nature. Power concedes nothing without a demand, and the limits of tech monopolies are prescribed entirely by the endurance of the people who build and use them.