Bitcoin not cryptos.

They do but what you are missing is the recession part. When you say "value [of fiat] dropping", that's inflation and that leads to more bitcoin adoption and its price going up.
But when we have recession, people tend to sell everything they have to get some money to spend and cover their cost of living. Hence bitcoin price dropping due to increased sell pressure.

We have to see how that affects the economy on a large scale. AFAIK this has never happened before so we have nothing to speculate this based on.
I believe the default would lead to increased interest rates and a massive recession which could cause a bitcoin drop but at the same time that would be US economy falling apart and people would also look for an exit of the failing economy and this could translate into more investment into bitcoin instead which could lead to the biggest price rise in bitcoin history.

P.S. Another thing to keep an eye on is "when". The world order is changing and the world is decoupling itself from US economy to some extent. In a couple of years it won't matter as much as it does today or it mattered a couple of years ago if US economy falls apart. That means the effects on bitcoin (which is global) would be a lot smaller.
P.P.S. In any case The effects will be short term, in long term bitcoin will become more valuable as it is the only "money" that exists in a world filled with uncapped "currencies".

It is not entirely clear if Bitcoin and other cryptocurrencies could completely replace the US dollar or other currencies in the event of a financial default. Some argue that Bitcoin and other cryptocurrencies could become a more stable store of value compared to traditional currencies, as they are not tied to decisions made by central financial institutions. However, there are also doubts about the ability of cryptocurrencies to function as a medium of exchange and unit of account, as their volatility and lack of widespread acceptance in everyday use may make it difficult for businesses and consumers to use them as a primary currency.
In the event of a US dollar financial default, there is likely to be a significant economic and financial uncertainty and the reaction of cryptocurrencies will depend on the decisions of investors and actions of regulatory institutions. However, it is important to note that Bitcoin and other cryptocurrencies are not a panacea for all financial problems and also carry risks

In  a way, I think it's good that BTC is not as independent of other economic factors like stocks (as I talked about yesterday also) or US dollar performance. So if US govt defaults (trust me it won't), BTC will crash badly like everything else.

That means to me that BTC is important to the same investors and same users of stocks and dollars and everything else. Unlike before when only nerds were using BTC

Doesn't that prove mainstream adoption?

News of the US government default is always talked about every year, but I have never seen it happen. The dollar is ruling the world and the US government owns the dollar printer. How can one think of the prospect that they will default?

But assuming that happens, the world financial system will collapse then that could be an opportunity for assets like bitcoin or gold, then gold and bitcoin will become everyone's safe haven. It is impossible to imagine what the bitcoin price achieved at that time would be, but unfortunately, that can only happen in dreams .

If the US government defaults, it will have a huge impact on the entire market but it will not kill bitcoin, because bitcoin depends on any institution or asset. Bitcoin is completely independent of centralized things like governments or US dollars.  And more, that could be a huge opportunity for bitcoin as people will need a new asset to replace dollars, and bitcoin will also be an option.

It seems to me that at the moment bitcoin is not ready to replace the dollar

I assume if the Unites States actually ends up defaulting her debt it would only for a short period of time before the politicians finally get in touch with reality and approve a new maximum.

If that happens, I would expect Bitcoin to go through a dump (not a very deep one) and then go back to these current levels.
For now, Bitcoin price is influenced by the economy of the United States to some extent and I expect those news to make some traders to close some positions on Bitcoin to move to other assets.

The US dollar is dominating the global financial system, so its collapse will surely severely affect all markets but then it opens the door for all the rest of the assets, and that can also be a big opportunity for cryptocurrencies, especially bitcoin.
If the US government collapses, I see more opportunity for bitcoin than the end of bitcoin because surely then people need to find an alternative asset to protect their assets. But honestly, the probability of US government default is almost zero.

If it can happen, let it happen, I very much expect the US government to default. When an old empire collapses, a new one is born and that's bitcoin's chance. Maybe people won't choose bitcoin as an alternative, but the future is unpredictable, and we have every right to hope. The centralized financial system will be replaced by the decentralized financial system, why not?

But is it? Point me to cases where people "woke up" and changed their currency preference due to fiat devaluation. Because as far as I know, when there is rise in inflation, most sell bitcoin, because they rather need more cash. And when the inflation rates drop, the bitcoin price rises. Which, as I see later on, you agree with. So what's that recession part you're talking about?

I'm of that opinion as well, but for the former to stop being a standard, some disaster has to happen I'm afraid.