I agree to a few of these points, but it's also part of that hitting coach's and his staff's job to identify the causes of, and solutions to players slipping into funks, especially whether it's a physical change, or a mental one; preferably before they go into season long slumps
I get there's variances and sometimes stuff just doesn't fall in for a week or two at a time, but guys like Vlad, Bo, Varsho, Kirk...all these guys have gone through stuff that isn't just explained away by a few weeks of random noise.
Injury probably also a factor for Bo, but a few calf issues don't sink you from a 5 WAR guy to replacement level.