Judy, I am going to say 8 years is actually a pretty good life for a tent. With that said, I know how hard it is to let go of a piece of beloved gear. Without seeing the tent, I can't say why the person in your REI store told you seam tape wouldn't work - though they are right, you can't replace just a rain fly, unfortunately.
I would find a liquid seam sealer (we sell a couple of options) to reapply to the inside of the rain fly directly over the stitching. The hard part is this, If there is still original seam sealer in place, you are going to have to remove it. If it is flaking off, it will just take the new seam sealer with it.
The only thing you have to lose is the cost of the seam sealer, and the time it will take to apply it. But if it works, a beloved, well-traveled tent will be around for a few seasons more.