I think there is a fine line between what a high-level, over-funded governement agency like the NSA, the NKVD or the mossad *could potentially* do, and what they *will* do...
It's not a tale of what's technically possible, but about which level of paranoid you are, and about the odds of being the victim of a (mostly) 3 letter agency.
I remember reading some leaked documents by Snowden, apparently the NSA hates Tor because they can't do much about it. Google "Tor stinks". Unless they have advanced in their methods to de-anonymize people, last time we know their opinion was that they had huge problems with it. As far as I know, there is no evidence of someone using Tails with all scripts disabled behind deanonymized ever.
M personally im not worried, since I don't do anything illegal, I just use Tor to hide my IP from the huge amount of scammers in the Bitcoin world constantly trying to dox and steal your coins.
Like you said, for small things they will not bother... if the owner of a mixer is not moving a lot of money they may not bother, but if in 10 years there's a mixing service moving $billions then he better have perfect anonymity because I doubt governments will just sit and watch as huge amounts are mixed.