Hi Katja,
felted wool is a natural material that wears down with use. Think of the elbows on sweaters. And it’s the same with felted shoes. For most people, the heel gets thinner after about a year. In your case, your big toe probably lifts upward anatomically when you walk, which is why you have that thin spot there. A lot of people experience this with regular socks, too.
With my felt shoes, I always sew leather soles underneath. That makes them last four times longer than without them. And the toe won’t wear through the leather that quickly. It’s a bit of work the first time, but you’ll get to keep your felt shoes longer that way. For the leather soles, I use old leather jackets. They yield dozens of pairs of leather soles. I also “punch holes” in them with pliers.
@Petra
I’ve tried many types of felting wool, and unfortunately, in my opinion, the ones from discount stores are inferior. You can tell just by touching them—they’re usually thinner, and you can pull them apart by hand. Good felting wool is really thick and shaggy. When felting, these fibers interlock much better and form a thicker, felted fleece. That’s what keeps your feet warm.
Best regards, Ina