How are most audiophiles going from streamers to DACS

In my experience, the answer to your question mostly depends on how the associated equipment is optimized.  I have used the USB, Coax, and optical connections that you asked about, plus AES/EBU and I2S.  In general, I2S was intended for very short runs inside of equipment and not as an external connector for two different pieces of equipment.  My current main system gear is set up to optimize USB so that is how I use it.  However, I use optical for my less expensive garage/outdoor system (Metrum Baby Ambre to Jade) and do not perceive any loss of SQ.  In cases where all connections were assumed to be equal with the gear being used, I have mostly preferred AES/EBU.  When using coax, I have mostly used BNC (and not RCA) connectors for reasons quoted below by the owner/designer of Empirical Audio.

I have explained this many times on this forum.  RCA connectors cannot be properly terminated to coax cable, I don't care what RCA you use. Coax cables have BNC connectors specifically designed for each type, so that the impedance discontinuity is minimized. Using RCA terminated to a coax will cause unwanted reflections, resulting in jitter at the receiving device. Adding a BNC to RCA adapter is usually harmless and does not result in compromised SQ.

Steve N.

Empirical Audio

In summary, try the different connections and use what sounds best to you.

I2S or AES/EBU. Metrum Acoustics Ambre streamer to Pavane DAC

I had that exact set-up with Pavane and then later with their Adagio. I agree that with those pieces either I2S or AES/EBU sounded best. However, for my tastes, my Mojo server and DAC connected by a short USB cable is sonically equal in every way and better in some ways. I doubt the differences are solely related to the connection method but rather the digital processing implementation, power supply, and other factors. The server uses JCAT’s highly regarded XE USB card and the DAC is also optimized for USB so, at least with this gear, USB sounds very good. This is why IMO there is no one universally superior connection format but rather it depends on how the partnering equipment is designed.

Make sure you are not mixing up fiber (i.e., optical) in lieu of Ethernet for purposes of bringing the digital information to the streamer vs. toslink (i.e., optical) as a means of conveying digital signal from the streamer to the DAC.
The first is in no way a compromise compared to Ethernet (although some will argue that it depends on the interface).
The second, optical toslink vs. electrical coax is discussed in this paper. BTW, Charles Hansen was founder/owner of Ayre Acoustics. If you click on his name at the bottom of the article it brings you to more discussion than you probably want.
In short, both toslink and coax can sound fine although coax is typically considered better...unless you start talking to folks who use USB.
Here is a post discussing USB vs. S/PDIF, which favors the USB interface, primarily because of the benefits of asynchronous mode USB, which slaves the source (i.e., server or computer) to the DAC.  However, according to the author, S/PDIF interfaces have  improved to the point where there is virtually no measurable or audible difference between the two, when properly implemented.  This is beneficial to those who still spin discs with a CD transport, which typically output through S/PDIF.