And if it is illegal just because the actual money is somewhat anonymous (which Bitcoin isn't, really) doesn't mean that the change in material wealth is unnoticeable.
That's just silly. You can certainly think of things people have a legal right to do that they still wouldn't want any record of. If you're paying for an abortion it's perfectly reasonable to want the payment to be truly anonymous. The same goes for other types of medical treatment, any purchases of an embarrassing nature, donations to politically charged organizations like Planned Parenthood, charity by people who believe altruism is only satisfied when the donation absolutely cannot be attributed, etc. There is also the simple metadata concern that giving anyone a record of everything ever purchased by you (and everyone else) could be used against you or for evil purposes generally.
The ability to make anonymous payments has been the norm since money has existed. Cash is anonymous. It's only the last few decades with the rise of electronic payments that tracking them all has even been possible, and there is no law of the universe that says we have to accept the loss of privacy.
"BTC" is just the unit of account powering the blockchain. You can use a blockchain to make almost any form of digitized information into a public good. Case in point: https://www.blockscan.com/asset
The blockchain is the ultimate distributed database. No other distributed database works without a third party controlling and administering it. This is critically important for all kinds of financial markets, because otherwise the entity in control of the DB is able to manipulate outcomes, hide data, and play games with insider information, and yes this is really happening: http://deepcapture.com
And why does a purely online-only, digital content provider need your home address to send you a video file, anyway? What about a server host? Gaming platform? The credit card paradigm is obsolete for a huge amount of premium digital content. This isn't to say the US dollar is obsolete, because you can represent dollars, or euros, or gold, whatever, as auxiliary currency units on top of the Bitcoin blockchain, although you'd still need to own BTC to send it around, for many this is BTC's primary form of inherent value.
Finally, it's impossible for many people in developing nations to engage in online commerce at all, because their banking system is utter crap. Billions of people the world over are underserved by the banking system, or don't have access to a bank account.
I think most people would welcome a shift to more anonymous transactions, but they're not willing to give up the advantages they get from tracking. When my purchase is tracked by a the seller, I get volume and loyalty discounts and easy returns. When my purchase is tracked by a credit card company, I get the ability to dispute payments and better manage cash flow. Most people and businesses aren't willing to give up those conveniences for anonymity.