do you think it'll change altogether to make the elders totally accountable for pioneers appointments some day?
It's already happened.
Since the beginning of 2010, or maybe earlier, don't recall exactly, pioneers are appointed exclusively by their congregation's service committee.
The elders then inform the Society of their approval via the JW.org website. Thus the Society knows who the pioneers are, but no longer appoints or removes them.
The timing of that change, along with the removal of the PAO program, seems to be too close to be coincidental.
I suspect you may be right, there may be some fear (or who knows, maybe a current lawsuit) of a pioneer being "responsible" for something unpleasant for a "trainee".
As you say, follow the money. No one is going to get rich suing a window washer or vitamin salesman who is also a pioneer. However, if a direct link can be made to the multi-billion dollar publishing corporation behind it all.....