Thanks to Jeremy's and the XOrg' team help and support it is now known that in fact IGLX is not going away in next releases but it is just disabled by default in the current 2.7.9 XQuartz release.
If you need to enable it for your work what was suggested and THIS DID WORK was:
Edit /opt/X11/bin/startx and add +iglx to defaultserverargs
I have tested that with Macbook Air OSX 10.11.5 and XQuartz 2.7.9 and it works with no problems.
Please try to gate this by a defaults option in the next release.
Thanks a lot for everyone's support!
This really makes a difference in our work!
*** This bug has been marked as a duplicate of bug 96260 ***