I have seen these subject treated in different posts before, but I still do not have a complete view of the results.
PI ACE is slowly disappearing and the lack of a VS 2015 compatible version goes in that direction, I suppose.
As explained by different users in other posts, there is a bunch of ACE use cases where the future Analysis Extensibility could not be enough, at least looking at what we have now.
The most recent case I have seen is a case where the results of a calculation depend on inputs coming from different branches of the AF structure and the outputs are sent to a WebService (I suppose that crawling the entire AF tree won't be allowed in the analysis extensibility coming).
Now the question is... is there any project to implement an "AF ACE" in the near future, or should we start to develop our ACE replacement? Having the information in advance can help a lot...
Something similar happened when notifications lost the Custom Delivery Channel feature last year.
This is taking us to delay updates and I have read recently that probably the feature won't be availalble in the next version either.
This is creating big problems with customers when are told to upgrade to the latest version, but then they can lose part of the work done (and paid).