I've thought about this a lot myself, anon, so I'll provide my three cents on the matter:
>but that's the problem with big tech, there IS no competition, and there never will be!
FOSS provides a good bulwark against this, which is why Big Tech keeps trying to co-opt/silence the free software movement every five seconds.
Look up (((ethical software))) and the recent smearing of RMS if you don't already know. With more and more people, both anons and normalfags, ditching Google and Microsoft, and with the accompanying upswing in FOSS generally and Linux in particular, we now have a powerful weapon in the resistance against Big Tech, one that is increasingly able to keep people employed and innovative. Things like the PinePhone and Freedom Phone, for all the worry surrounding their actual nature, are nevertheless a great sign, as they indicate market trends moving in the direction of popular distrust of the state and its corporate vassals.
However, the most potent thing to remember, as
>>4565 mentions, is that bailouts are just as crucial to the Big Tech oligopoly as they are to any corporate hijacking of the market more generally. Thus, in an unregulated economy where the little guys aren't being taxed/regulated to death, and the big guys can't rely on bailouts to save them, both will have to rely on innovation and pro-worker, pro-consumer practices, or risk taking serious losses. I
would like to think that this will have a domino effect on the shareholders, too, at least to the point where investors are eventually less likely to invest in reckless companies, since they would, as time goes by, experience diminishing returns in doing so due to the aforementioned reclamation of the economy by small businesses and consumers.
Again, FOSS is absolutely key here, as are similar endeavours such as the right to repair, open-source infrastructure, and the cryptoanarchist movement.
Remember, too, that in a proper voluntaryist/ancap/agorist/whateverthefuck society,
covenant communities would ideally be as self-sufficient as possible. I could absolutely see autonomous communities relying entirely on FOSS technology, since they would likely only need to use it for the purposes of sales and infrastructure, both of which are easily doable in that vein. Actually, in the formation of ancapistan, such technology would undoubtedly be instrumental in evading government loyalist surveillance. Proprietary garbage would only really need to be used in certain specific cases, and even then, would likely be optional.
TL;DR, Support free software, deny ad revenue to the big companies whenever possible, set up alternative economies and infrastructure, and push to equalize the economy by removing both taxation and bailouts.