you are making two big wrong assumptions.
the first being bitcoin is only being used in darknet, which is wrong since that usage is very small and negligible. and instead the rest of the usages you listed has been growing.
For me, darknet meass, any payment that is made without traceability. All bitcoin transaction can potentially untrackable and hence fall into darknet category. Untraceable money movement is plain illegal for two reasons: it could be used for money laundering, or it could be used for bad things.
You don't even know how Bitcoin works. It's not anonymous, it's pseudonymous, and blockchain analysis tools from governments are quite good. If used properly people can sort of reach anonymity, but it's quite difficult.
It's important for you to understand that drug transactions happen with or without Bitcoin, and that even now the vast majority of illegal transactions are done offline.
the second assumption you made is that people are using bitcoin just because they don't want to pay taxes! first of all there is nothing wrong with paying taxes, if you have problems with that then you have problems with some fundamentals of society.
Society has trust problems. That is why bltcoin uses mining and blockchain. Now you are telling me to trust the society to pay their taxes, while they can easily evade them.
They can't "easily evade them". What will happen if Bitcoin grows is that exchanges are regulated so that any transactions converting to fiat are subject to capital gains tax. As for VAT, it wouldn't be affected very much. People pay in cash at shops, but because the
shops are regulated, they still pay VAT.
secondly there are a lot more to bitcoin than just "tax evasion"! there are lots more. things like removing third parties as @Xavofat mentioned. things such as reversing the "trust" part between merchants and customers as the white paper mentions. or the fact that you have 100% control over your money in the safest manner possible.
Or in other words, you don't know how to deal with it.
Deal with what? Sovereignty of funds is the most important thing in a cash system. Moving into a digital age, you need to have a system which doesn't rely on someone else to hold your funds for you. You can hoard gold in your house and you should be able to do the same thing with digital money.