Bitcoin being accepted by the mainstream would be pretty nice. That would mean that the price of Bitcoin will be pretty high because there would be more people are using it to make payments and that such.
If Bitcoin does get to the point where it's worth a bunch of money then I kind of have a feeling that Bitcoin's Blockchain will slow down because there won't be as much people wanting to spend to coins online or offline for fiat.
I think you've illustrated perfectly what "hinders bitcoin to become mainstream":
there's no need for it. Bitcoin doesn't offer to the mainstream, any marvellous possibility that they don't already have: be able to pay for things on the internet. Most people buying stuff on the internet, have credit cards that allow them to. Most people that could use bitcoin, have bank accounts (fiat bank accounts) that they use for receiving payments and sending payments. Even though it costs some money to do so, this works very well for most people, and bitcoin doesn't solve a problem for them.
The fantastic possibility that bitcoin offers: "paying people on the internet", has already a widely used solution for most people, a solution they know, works well, and they are comfortable with: their bank and their credit card. So why would they use bitcoin ?
The very fact that you say that this will make bitcoins very expensive, means that you are essentially (like most people) into it for "gains".
But what are gains in bitcoin ? They are: buying bitcoins cheap (and hence, early) to sell them expensively (and hence, late) to the last layer of adopters:
a greater fool game. Greater fool games come down when one runs out of greater fools.
Of course, there is the possibility of a global financial crisis that brings down the entire fiat system. THEN, of course, people will not trust their banks and their credit cards any more. Then bitcoin or other crypto will offer them something they can use, and that solves a problem for them.
But as it is now, there's not much in normal usage that can create a demand for bitcoin by most people. If you tell them that they can stop using their bank, but now they have to go to an exchange on the internet, do a registration with AML/KYC rules, send their fiat money there, to get bitcoin, do finally their payment in bitcoin on the internet, what's the difference between a bank that is strictly under legal scrutiny, and an exchange that is somewhat more risky ? You have to use your bank, an exchange, and bitcoin, and pay fees on all of them, instead of just using your bank. What good is that ?
There ARE applications of crypto: where fiat cannot go. But most people don't go there either.
And yes, the greater fool game of "buying bitcoin now, when there is still low adoption, and sell it later, when there's full adoption" can still go on a long time. But if you see that almost everybody already adopted bitcoin, why would you join ? And who will pay for it then ?