Since I donated, my remaining kidney has grown larger and taken over a chunk of the function of my missing kidney. If possible, getting it back would only give me a relatively small benefit compared to the risk of the surgery. First, I'd have a spare again for major injuries meaning I'd be 'allowed' to do things like contact sports and such. It's recommended you don't do things that could result in an impact to your kidney when you don't have a spare. I did violate this once by going skydiving, though.
Second, I'd be able to take anti-inflammatory meds without restrictions. That is an annoying one as when I injured my rotator cuff years ago, the normal recommendation was a month of high-dose anti-inflammatory meds. Since I couldn't do that, I had to do physical therapy and injected steroids which was much more involved and more expensive.
Note that a transplanted kidney can be re-transplanted depending on the health of the recipient when they die. It's not as common but it has been more in recent years. It's often referred to as "regifting".