I think you are missing a few key things though
1) Before vaccinations, Marek's disease disease was a HUGE problem, killing (or damaging to the point of making them unviable) a significant number of chickens. These days, with vaccination it is pretty much a non issue as long as you vaccinate for it. So it's a HUGE net gain
2) It would be ideal if we could've wiped out Marek's instead, but that didn't seem to be happening. The vaccine was created in the 1970, but the disease was named after a guy who first described the symptoms of it in 1907. After 63 years, it was still killing chicken...but maybe if we had just waited a few more years?
3) I'm not clear if you are aware of this or not, but FYI...the nasty version of the virus was not created because of the vaccine. It had existed before the vaccine, but it killed the chicken before they reached the contagious stage. The vaccine allowed it to spread more easily and helped select it.
4) In light of #3 above, this is what makes the whole Marek's disease discussion irrelevant to any form of covid we currently know. None of them kill before being contagious. In fact, in the vast majority of cases, the most highly contagious phase is over before serious illness begins to set in (which is generally a few weeks after the host became contagious). People bringing this up are merely grasping onto something that is slightly tangentially related and acting like it's vitally relevant. I see a lot of that from the anti-vax crowd these days.