I find that high quality medium weight merino lasts for decades. I have a few 20 year old Merino skiing mid-layers that I wear all winter that are still holding up fine. But when you get to the very lightweight merino I find that it falls to pieces after a few seasons. But still worth it. A lot of modern merino sports wear is actually a blend of merino and synthetics, which actually makes it better overall if well designed. My Rapha merino jersey is blended and doesn't suffer from the sagging issue mentioned up thread. The outer layer is also a little smoother and doesn't catch as easily.