New Web 2.0 applications spring up left, right and center that gain up to hundreds of millions of users in just months. Think Tik Tok, for example. It was launched in 2016 for the Chinese market and in 2018 for the international market. Today, it has a whopping half a billion monthly active users. Web 2.0 is something that the masses are conceptually very familiar with. Development tools for responsive and fast web applications that are extremely scalable are reasonably mature.
I wrote a post in which I discussed why it seems that Web 3.0 apps have been so slow to take off. I consider the fear of sovereignty to be one of the main reasons why it is taking so long for people to get onboard with such an obviously superior alternative for a blogging platform as Hive where you can immediately start earning cryptocurrency for your activities. (If you really think the rewards are peanuts on Hive, then I suggest you give any mainstream platform a try.) Because the fear of sovereignty and the longing to have an authority figure hold one's hand is a philosophical or even a deeply rooted temperamental issue, then the implication would be that the ceiling in terms of the theoretical maximum number of users is nowhere near the four billion Internet users but an very much smaller number.
The Libertarian Cato Institute reviewed studies done on how the political views of the American electorate are divided along social and economic lines and their conclusion was that 7-22% of Americans consider themselves libertarian. The United States is much more libertarian than most other countries, though. But it should be remembered that those living under dysfunctional governments are likely to find sovereignty acceptable than otherwise because they have no other choice.
The question is how many people either want to adopt Web 3.0 on philosophical grounds or at least do not find it unpalatable on those grounds or are in such dire need of additional income that they're willing to put up with being in charge of their own funds and their data. If the lower bound figure is used, then 7% of four billion Internet users would still be a very large number: 280 million. While DApps may never become the most popular apps on the Internet, the potential user base is still very large. If the user experience can be improved sufficiently, then there's chance decentralized apps could become hugely more popular than they are now. If Hive has 10,000 monthly active users now, then 100x growth would only mean a million monthly active users.