Thanks Anna. It still means people looking for resolutions to problems are going to have to search across a number of sites and potentially sign-up with differing credentials... (Some use MS accounts, some use Facebook, some user others...). I understand and appreciate the different focus' of each of these sites, but the reality is that the differences are essentially lost/meaningless when people are trying to resolve issues. The first place that comes up with the closest answer, is likely to be where the conversation continues, regardless of appropriateness.
A person bug-bear of mine is SharePoint 2016 and SharePoint Online. There's a thriving community in the TechNet/MSDN SharePoint 2013 (and older) forums. But SP2016 has been dropped from the MSDN forums and there's only a single SP2016 forum on TechNet. SPOnline "technical" focus seemed to move to Yammer (Office 365 Network), which is now moving to the new Microsoft Tech Community and has a Developer sub-forum, which kinda goes against what you've stated above.
I think if you're going to continue with quite separate support services, that are partitioned based on focus, rather than just product, then this needs to be enforced and processes need to be put in place to push questions/content/users into the right environments. The different focus' need to be clear and obvious, with perhaps some kind of unified nav/guidance to point users in the right direction (e.g. the Community Nav in MSDN should include links to other support services) - I see TechNet tries to do this, though it's "Other Microsoft Communities" links now just go to answers.microsoft.com
Sorry, just a bit of a rant from someone who already has too many RSS feeds, too many communities, too many support sites, too many emails...