First requirement of going mainstream as currency - solving the scaling problem. Otherwise Bitcoin will be a "digital gold" - expensive to move, suitable mostly as storage of value or used only for very expensive purchases.
We may be getting close to it as offchain and sidechain solutions are being developed, so this problem might be solved in a few years.
The second problem is regulation and competition - some governments might start actively opposing Bitcoin, banks will start offering their clients better deals to prevent them switching to crypto. Average people probably don't really care much about decentralization and financial freedom, they want fast and cheap transactions, which was the original promise of Bitcoin, described in Satoshi's whitepaper.
Totally agree with your point of view. If Bitcoin will remain a kind of digital gold it will never become mainstream, because it will be used just for investment purposes and investors (in any market) are generally a minority compared to the whole world population.
So Bitcoin can get mainstream only if it is used as a currency and a payment method. Otherwise, if Bitcoin will be used just for investments and speculation, it won't get mainstream. Never.
I think bitcoin can go mainstream two different ways. Either it becomes the investment of the 21st century, or it is used as a currency.
As an investment, some kind of digital gold, I don't think it is really necessary to solve the scaling problem. People do not buy or sell gold every day.
In that case the number of transactions would not increase that much. And if you invest, you have more time to wait until your transaction is done.
Bitcoin going back to be a currency, then we need a solution for the scaling problem. And I think that would lead to many disappointed investors as well.