Bitcoin price is first and foremost measured in fiat, mainly the dollar.
This is to be expected since fiat is still the common unit of account for every good on earth. This can change, but before this Bitcoin needs to be an estabilished store of value and medium of exchange. Its just the natural evolution of money. Its only a practical unit of account when the world adopted it. We gotta be patient here.
Bitcoin is bought and sold with fiat since the beginning. Was that a tragic misstep in the evolution of Bitcoin?
Nope, its the process every good that wanna become money has to go trough. Most people wouldnt even have the chance to acquire Bitcoin otherwise, leading to no significant adoption. Theres also no practical benefit to account in Bitcoin right from the beginning, when only a tiny minority does it, so patience, this will change with adoption.
Would we have it better if all governments outright banned the purchase/sale of BTC with fiat?
They cant really ban it, they can prevent services to build around Bitcoin, but thats it. It would have probably slowed things down. All the bad these exchanges are doing is also a reason for more people to improve their security practices and a real world example of what centralization leads to. Its like an immune system that adapts, it becomes stronger if you challenge it regularly, so nah i dont think it wouldve been better.
If we had a fiat-decoupled version of Bitcoin which could be traded for services and goods without a third party currency, how would that look? Obviously the government would have no power over people accepting BTC for their goods and services (other than intimidating people with punishments, if caught).
We have this fiat-decoupled version already, because it never existed in the first place. The common money is still fiat so it doesnt have any advantage to not be able to get Bitcoin with it. Its about what people are accepting in exchange for Bitcoin voluntarily, this is always a individual choice, not a set rule. There would always be people accepting fiat in exchange for Bitcoin,so there will always be a market to exchange fiat and Bitcoin, this happened naturally and couldnt have been differently. In the future people can just use satoshis instead of fiat to price things, there wouldnt be much difference to how it works now.
In such a reality, I think the purchasing power of Bitcoin would see the same rise in value from its inevitable adoption but without the dumps caused by the broken fiat economy and stock market crashes.
We still live in that broken system, Bitcoin is designed in a way to use the weaknesses this system has, to its own advantages. Let the dumps happen, we will beat them at their own game, thats the only way. If we wanna beat them, theres really no way around the pain we have to go trough now.