What you're saying is right, Governments will find ways to tax cryptocurrencies one way or another. But two things could happen, firstly governments could render cryptocurrencies worthless due to over-regulation, in other words destroy the reasons why these are so useful i.e. privacy, low fees, real time, etc. thus destroying the blockchain world. Or they will hopefully realise that cryptocurrencies do provide a useful service and so they strike a reasonable balance between regulation and freedom. There's a third thing that is already going down the gurgler, governments could do next to nothing. But this won't happen.
Hopefully governments will find the sweet spot which enables cryptocurrencies to flourish and perform while keeping officials happy. I'm holding my breath but ...