In the UK mycoplasma is endemic. In the West Country where I am it is believed the vast majority of chickens are carriers. DEFRA and many vets used to, and maybe still do, recommend culling those infected.
More recently many chicken keepers have rejected the culling advice, the reasons being it is so prevelant that most flocks would have to be culled if the expert advice was taken. More importantly, if one culls the infected then the ability of the species affected to manage the disease and build resistance to it is taken away to a great extent.
It's a bit like the advice we got during the peak of covid. While we didn't kill everyone who contracted the disease the management of it was of a similar nature, vaccinate and isolate.
Well, covid is still here and most of us have survived it. It's much the same with Mycoplasma. It becomes critical in times of stress and ill health, but otherwise the chickens live through it and hopefully in time they will build some immunity to it.
Mareks is much the same. Fayoumies for example are said to be immune to Mareks and there have been reports from China and some other regions of Asia that mortality from the disease is declining.
Culling chickens with Mareks is in my opinion very short sighted. There are chicken keepers on this forum who have Mareks in their flock and while some of the flock die,others do not.
We don't kill humans when they get infectious diseases why would we treat chickens any differently?