I agree with everything you said except for the turkeys being short lived. Not all soils carry blackhead. I've kept turkeys now for 10 years and I don't have a problem. The turkeys run on the same range as the chickens when I let them out. Sometimes you won't know if blackhead is a problem in your area until you type some.x2 with Oldhenlikesdogs
Over the years, I have had what I strongly suspicion was Marek's in a bird here or there. (Since necropsies here cost nearly $100 to send in to the local lab, I did not have them necropsied...sorry...money was tight at the time).
I chose to breed from those that did not show symptoms those years naturally selecting animals that have natural resistance to the strains in my area.
Marek's is a morphing disease and it can create different symptoms depending upon the strain.
The good news is that most backyard owners keep diversified flocks. Some breeds are simply more resistant than others, so it is unusual now for a flock to be totally wiped out by Marek's.
As another poster stated, the Marek's vaccine is actually creating more problems since the vaccine only covers certain strains. The virus is naturally being selected now for the more virulent strains to survive. But that too will be short term as especially virulent strains kill the animals quickly and thus die out....but sadly if it was your flock that year, it doesn't help you much.
And as Lazy Gardener mentioned, the old timer trick was to raise some turkeys along with the chickens. Turkeys naturally carry the turkey strain which is much milder but gives immunity (or resistance) in chickens for the chicken strain. However, the turkeys will be short lived, generally, as chickens can spread back histomoniasis (black head) to the turkeys from the cecal worms in the chickens.
So Marek's is not the end of everything in your flock. Breed from those who survived with no symptoms or few symptoms. Over time, you will have a flock naturally resistant to the strains in your area.
LofMc