If we was to take the standard issue advice on most of the principal decay fungi, and actually follow it to a T, we would not make any prognosis without fully evaluating the situation. The reality is that very few individuals responsible for making these decissions are fully aware of mechanics, decay and fungal ecology let alone the full life proscess of a aging/veteran/ancient tree. There are many many more shades of grey more even than fifty between the fell all and the retain all camps.
If I was only going on the works that are currently avaliable to me to make my prognosis in the HUNDREDS of cases of principal decays i work I would have had to rely on others work and info and as a consequence a great many trees may well have been needlesly felled.
I have gone to great personal lengths to study the ecology and life cycle of trees and their fungi, and I very much take it for granted how easily I make prognosis or choices regarding retention of compromised trees.
I guess if there was a golden rule to this kind of evaluation, then it would be to base ones prognosis on the mechanics above all other matters, that applies from root to shoot and then concern regarding the potential invasiveness of the "parasite" the latter matter being a long way from adequately researched.
Meripilus is one of those I would not treat with less than the utmost of respects, it is one of the most capable of decay fungi, not only is it very capable of breaking down Pectin in the middle lamella it has both soft rot and white rot capacities.
that said, it is in all cases the conditions the trees grow in that is the major contributory factor, and hence we see where man has made the rooting environment difficult we see the most progressive and dangerous cases.
One reason for this problem is that beech are naturaly very shallow rooted in old age their heart rooted natures have long ceased, much of the time meripilus simply lives on the old root system that is no longer functional, it is only later and increasingly under stress conditions (compaction) that the fungi must move into living tissues and become the enemy we so far understand it to be.
If you study Meripilus youll soon see the direct correlation in these points I raise, of course some wild trees fail but they do not fail nearly as quickly as those in the urban context, because of the conditions.
We must ALWAYS remember that ensuring healthy growing conditions, from the Rhizospphere to the growing tips is the key in these situations. A tree growing in optimal conditions will compensate and cope with colonisations even of the most feared of fungi with relative ease, but in stressed conditions, especially drought fungi will win the day everytime.