Well, I am not sure we can be that certain.
It is a fact that magnetic tapes do degrade over time, but I would like to share my experience on this.
I personally own the Dinah Shore show with J5 performance. I do not recall how old it is but we can reasonably say its 20 years old.
Believe me, quality is JUST great. The tape I bought was in a mint state but surely deteriorated over time. Still it's very pleasant to watch. TinoMJ posted a rip of this tape in the 2000watts section.
Regarding the snippet posted by the estate, are we even sure it's a direct rip from the concert VHS ?
As far as I know the estate never confirmed nor infirmed it. Could be from any other source, even from another YT video should they be in a rush to build the trailer. This snippet of bad song from wembley can be found in many TV special, news report, fan videos etc...
And finally, I was watching PHM recently. You can see snippets from wembleys show. I recall a very short snippet where MJ is dressed with TWYMMF outfit, fainting to punch the camera. Surely from personal MJ video collection in VHS and quality is pretty I must observe.
Again, as many of us said, let's wait for the DVD to be released but I am confident we'll really be blown away !