I"ve been drawn into religious arguments in the past (and got blasted accordingly) so I want to state right now that I don't count the Way as a religion (which is why I called it spiritualism instead). Prior to the invasion there were may spiritual groups within the Clans that would help feed the Cobras (Clan Nova Cat, Clan Coyote, Clan Goliath Scorpion (hopefully I got that one right this time), and to a degree Clan Ghost Bear). The strongest spiritual sources have since moved to the Inner Sphere and with contact completely cut off, that source is cut off.
My second point is that isolation and specialization may create great warriors today, but as the world changes around you, you will soon be out of date, and out matched by evolving generalists. You only have to look at human evolution to see this. Specialist hominids (like Homo Boisei) were able to out perform generalist hominids (like Homo Habilis) in their hey day, but when the environment changed, the Boisei went extinct in less then a couple of generations while the Habilis struggled on for another million years. The same is true for the Clans, clans like the Blood Spirit create exemplary warriors, but their isolationist attitude will catch up with them in the end (as shown before they started to develop Protomech technology). Generalists like Clan Wolf will change with the times and survive because of it.