My experience is that the clear stick-on plastic chainstay protection lasts for decades without visible deterioration. My late 90s Riv custom came with one installed, and it's still in place without visible damage - in fact it's very hard to see unless you look closely. And on my Bleriot, as well as chainstay protection, I also fitted some protection along the top tube, because the bare brake cable runs at about 2 o'clock along there and my leg presses it into the tube whenever I stop at an intersection. So I protected the paintwork shortly after I started riding the bike (about 15 years ago) by sticking some transparent plastic along the top tube where the cable would rub, and it similarly is pretty much invisible unless I point it out to people.
I didn't buy the stick-on frame protector kits that bike shops sell. I went to a motorbike shop and bought several sheets of the 3M film intended for paint protection (each sheet was about 12" x 15") and cut the sizes and shapes I wanted from that. I still have a couple of the sheets left, waiting to be used.
Nick