OK fine, how about if I am at Starbucks, using a disposable phone, while running Tor.
I made the transition in 10 minutes, then I'm out and never return. Now what?
It's very hard to keep your identity completely hidden. Probably impossible to have it exactly 100%.
There are cameras that could be used to facially recognize you. Your disposable phone, unless it has all antennas removed, will send and receive information from and to the service provider it's been registered to (and even without it, AFAIK, a phone with battery in it could still send and receive information - especially if it's a smartphone, and assuming you're using
Bitcoin with it... it most likely is). Just think about it - even without a SIM card you can still call 911, right? Even if you turn your phone to Airplane Mode, do you have any
guarantee that it has terminated any communication?
What is the source of your Bitcoins? What if your government or a third party has been doing blockchain analysis for a long time and your identity has been easily found? Mixers do not hide your identity 100%. While it hardens the analysis process and makes tracking your coins a pain in the arse, it does not mean it's
impossible to find who the owner truly is. With enough resources and time, someone looking after your BTC address may link some crucial information to find your identity.
There's more to it. It's just that, while you do improve your privacy, you never push it to perfection. There's always going to be some leakage you may not even be thinking about, yet important enough for authorities or someone tracking you.
Edit: Tor does not hide your identity either, if you do not use it the right way. First of all, a lot of people use it with JavaScript enabled - which is just enough for your real device's information to be leaked. The thing is, you're mistaking anonymity for pseudonymity. It's not
anonymous. It's not like I send you 0.001BTC and nobody knows who sent money to who, or how much. The ledger is entirely public, which means that once data is stored, it's going to be there
forever. In other words, any little mistake is going to be recorded without the possibility of reverting it. It's just like our usernames on here: you have usernames, you're not sure who is who.. but with enough research, you may find out. We're just pseudonymous -
not anonymous.