> For a proper financial market with money, a good store of value might just be a long term savings account in a bank

Not sure. Look what 2008 happened: States had to rescue banks which would have gone bankrupt otherwise and because of this the entire economy/currency suffered. A FIAT saving account—even in a developed country/financial market—is a terrible store of value.

> weak economies have more people who can't afford the rather extreme transaction fees

When I look at Google Trends https://trends.google.com/trends/explore?q=bitcoin I see many weaker countries or low-income countries (South America, Russia) which have a quite high interest in BTC.

The transaction fees don't hurt if it's just about putting your savings in a safe spot. The people are not going to buy groceries with BTC.