I totally agree with you, but the Blue Devils are a bit of an anomaly. Most DCI organizations don't have 4 fully-funded marching units (A corps, B corps, C corps, International Corps), and all the other peripheral activities (Wind Symphony, BD Entertainment, WGI Winter Guard, etc.) that the Blue Devils have. Having such a large organization allows them to have more full-time and part-time employees than most drum corps, which certainly allows for more mentoring and passing on of the methods and skills for continuing tradition and advanced show design. BD has made their transitions smoothly because of this, and they are to be commended, but most drum corps are lucky to just have 1 corps, 1 staff, a good admin team, and solid alumni and fan base.
When the Cavaliers lost Gaines, Saucedo, and others, they likely didn't have a "second string" if you will, learning from the masters and waiting for their time. So the loss of key staff over a period of time for most corps can be devastating. There are instances of it working super well, too. But take a look at SCV. They managed to grab Phantom Regiment's brass arranger, Phantom's percussion staff, and the Bluecoats brass staff -- all within 2 years. Many thought they would become a super corps. Well, I do think the marchers are super, and their shows are very cool and I love them; but competitively they have not made that push to top 3, top 2.
Good teaching matters. But chemistry among a staff also matters. Getting talented and hard-working marchers and musicians matters. Having funds matters. Having facilities and good instruments matters.