Nothing lasts forever, Jake, including boot soles. It sounds as if you’ve put your Limmers through pretty hard service, so it’s to be expected that they’re showing some wear and tear.
The good news? Boots like that are eminently rebuildable. The reason I say that is because the leather in the Light-Weight聴a single piece of 2.8-millimeter waxed leather聴is as heavy as the leather found in most “heavyweight” mountaineering boots. So while the soles may not last more than five years under hard use, the leather should last for the 20 you were led to believe. So I’d definitely send them in to Limmer, pay the $75, and have them do their thing.
And keep doing it. I had some Galibier boots made in France that I think went through four soles, plus some heavy interior reconstruction. And there’s a company in Spokane, Washington, called White’s that makes a boot favored by wildland firefighters called the Smoke Jumper ($350) that can go through seven or eight complete teardowns. And, I mean, the White’s people literally take the boot apart, replace any worn leather, and stitch it back together.
Good luck!