What percentage of Bitcoin use is legitimate? I wasn't able to find actual data on this. I'm under the impression that


the majority of transactions represent ownership transfer between investors, traders and exchanges
the majority of remaining transactions represent purchases of goods and services
the majority of purchases made with Bitcoin are for something illegal


It's very easy to see why someone would want to buy drugs with Bitcoin. It's not easy to see why someone would want to buy coffee with Bitcoin. If I want to buy coffee with Bitcoin, isn't it easier to first exchange that Bitcoin into fiat currency, and pay for coffee with that?

Does anyone have any data regarding what percentage of Bitcoin use is legitimate?
Silk road wallet hold 144,000 Bitcoins. It had 1,2B USD in annual volume. This was 2014.

Since then darknet markets have flourished. See Measuring the Longitudinal Evolution of the Online Anonymous Marketplace Ecosystem.

This is mostly for illegal drug related and other purely criminal activity like payment fraud (stolen credit card). This does not include ransomware bitcoins (1B USD). This does not include e.g. bitcoins involved in tax evasion and asset hiding. This does include bitcoins lost in exchange hacks or stolen from exchanges. Furthermore, most peer-to-peer and remittance trading could be accounted as money transmitting business and against local regulations. 

Bitcoin transactions are not very useful in casual purchases, thus there has been little mainstream consumer adoption. Bitcoin shines in anonymous online payments and most day-to-day and/or point-of-sale payments don't require this level of anonymity or the complexity it brings along with it.