Mostly the reasons mentioned in the posts above mine.
1) It's reaching critical mass. Proprietary internet is mostly made out of huge websites with huge server farms that require huge maintenance costs. A single user doesn't earn the website a lot, so serving a user must be cheap. Currently companies are able to keep server costs low but they're steadily rising, and once a large economic blow or even merely a shift in trend causes server costs to go up to a level where the companies can't make a profit anymore, the entire thing will collapse.
2) People are getting tired. Even for the barely-sapient 90% that we like to call the "NPCs," the constant decrease in quality for these services is getting noticeable. There's only so much bullshit they can take before they simply stop using the declining services. Also, a subtle but noticeable anti-internet sentiment is rising among the gen-Z crowd. Once they overtake the millennials as the general adult population, the numbers for the tech companies might start stagnating due to this. While this will not instantly destroy them, it'll make it easier for the first reason to do so. It'll also contribute to the rise of the free internet.