I don't find this to the the case at all. Most of my base layers are blended merino and they need to be washed just as rarely as pure merino. Perhaps not all blends are created equal.
Interesting what percentages are we talking? Something like 30% is enough to ruin the garment for me. Maybe I am just very smelly!
So I looked at my clothing, and most of it seems to be 87% merino wool, which is less plastic than I expected, however in my experience that 13% makes a huge difference in terms of durability.