They need to do significantly better here. The few times that I had to go to a service center (one was for a damaged rim and tire) the loaners have always been terrible. They were always either filthy and in terrible condition, or very old models that don't have anywhere near the capabilities or features that my personal car has.
At one point, I even went in to do a demo drive of a Model X that I was considering purchasing, and they handed me the key card to one that didn't even have working AC! Can you believe that!? How do you expect customers to want to purchase your vehicles when that is the experience you are providing. So I tested the car and suffered through the 30 minutes of the demo with no AC in the middle of summer.
Then there is also the case where sometimes I ask the service center if it is possible for them to loan me a Model Y. For no particular reason other than just wanting to experience the difference between it and my Model 3 for a few days to see what it would be like to live with. And each and every time I get turned down, and sometimes even get a downgraded car from my own.
Needless to say, they seriously need to address all the shortcomings of their service centers. What good is it to have a great car, but once you own it, the service experience is so terrible that you feel like they just wanted your money. And now that they have it, they leave you high and dry. To end, it isn't just related to loaners, but also to the general service experience. From the long appointment windows to even get service, to often having to wait days or weeks for what you would think are simple repairs. The entire thing needs a rework from the ground up.