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.
i think with all of these problems presented, especially with scalability, im of the opinion bitcoin itself wont be going mainstream. if anything, a new cryptocurrency based on bitcoin that aims to solve these issues with long term use and scaling in mind will probably be the one to take the torch and be the first real result from the experiment that is bitcoin. who knows, maybe not even that hypothetical cryptocurrency will go mainstream, it'll take a lot of time before we have anything with all the right attributes to actually implement into a large scale economy.
no because it has already happened in some countries like Japan and it has already started slowly and on a smaller scale on some other countries. so your question is somewhat moot now.
The price we dream of for a BTC to be 5 figures is the day my mum has her own BTC account which she takes travelling and she 55 years old.
it doesn't matter how big or mainstream bitcoin gets, some people will never use it. the same way they are not using other technologies such as computers, cell phones, credit cards,... i hear some old people who say they don't trust or understand them so they don't use them. it will be the same with bitcoin.
also true; no matter what, people will resist change, especially change in something that has remained unchanged in their lives up until that point.