No because it is understood that in the long run, the increased use means a coin with a very bloated blockchain that will become increasingly difficult to transact with, or have a properly syncing wallet.
The idea of disposable blockchains / altcoins, that serve a role for some time and then die, should not be ruled out if scaling issues with bitcoin make tx spillover into altcoins.
If not, why won't one get developed & superseded BTC?
I mean if we get to the point where people start transacting with other chains instead of BTC, so other chains will get the problem that BTC will be avoiding. Yes, at that point, it will be "understood" that this is so.
Yes, because that won't affect the price day to day. It will be a low price, a low marketcap, but it will be ok for transactions, or pulling money once a day or week. Just not long-term storage of wealth.
If you live in a weird country, you can buy btc out of a kid that has been mining shitcoins with his GPU. You can buy dollars from the bank - if the government allows you, and in countries with currency restrictions, this won't be possible at all. So buying BTC could be far easier than buying USD, withour reverting to the black market and paying a very high premium.
Theoretically, yes, if you have BTC it's easier to go to USD. However, you can't do much with the USDs in your bank account. They are presumably there, but if you go to the ATM and click withdraw, you'll get local currency at the running conversion rate.
A country will typically allow inflow of currency into USD bank accounts, allow you to hold there your incoming wires and stuff, but what you see as 1000$ in there, isn't actually there. These USD are used by the government as "foreign currency reserves". If you want them in your hand, they won't give it to you - you'll get the local currency equivalent, at the running conversion rate. What you get instead, is a digital number in your bank account (1000$) and that's it.
The obligations of countries to pay back the foreign currency to the foreign currency account holders has a history of not getting honored, as various countries spend their foreign currency reserves and then forcibly convert USD accounts to local currency. You never got to get them in your hand in the first place, but then even the digital 1000$ are converted to local fiat.