Most of the questions are the same (configurations, non-programming) and it would benefit from having more people looking at questions. Some of those sites don't get a lot of traffic, the response rate is slow, and have many unanswered questions.
I feel (entirely biasedly) that this proposal seems to miss a lot of context and community culture.
First and formost, most of these sites are successful by Stack Exchange standards and have met all the requirements for graduation. Where they didn't, they still have large self sustaining communities capable of answering questions.
Lets start with 2/3rd of the trilogy - Serverfault and Superuser. Superuser's all about the 'pet systems' individual systems, stuff you run at home. I prefer serverfault for 'big picture' things - infrastructure, servers and such. Something like this belongs on SF as does anything about running IT and services.
Askubuntu is a Q&A site officially supported by Ubuntu, and was an early and rather successful experiment that didn't quite get repeated. It has its own vibrant community and serves the wider ubuntu community pretty well.
U&L is interesting. It does fit quite neatly into the scope SU and SF have but in many cases the community there's really good at very difficult problems. SU's pretty broad, and sometimes you need depth. You're going to have to exercise the peewee herman rule in deciding where to go.
I've never been to ask different, but at the very least, while they intersect with our scope, they cover things outside ours (like iphones and apple watches) that arn't served elsewhere and probably have the same sort of depth in their subject matter U&L would have
I'd like to address a few statements made in the question specifically.
Some of those sites don't get a lot of traffic
and have many unanswered questions.
Once again, needs context. Not all questions are trivially answerable. I suspect I'd have better luck with some of my questions on U&L than the much bigger superuser.
SO has 3,671,063 to 108,223, though it would probably be fairer to look at the proportion to our total number of questions.
Luckily, there's actually a scoreboard of this here - SO has 72%, Askubuntu has 66% we have 68%, as does ask different and U&L has 77%. So with some leeway, we arn't doing that much worse than SO. (Workplace and parenting have 100%. Darned overachievers ;) )
Most of these are the top tech sites and have a reasonable amount of questions per day. AU actually has more than ours.
Superuser and Askubuntu are the second and third busiest sites on the network
the response rate is slow
That's relative. I'd like numbers there, and its worth considering that some questions that are actually hard take time to answer.
That's ... kinda narrow minded. We cover hardware to an extent (which U&L and the others don't). While historically we've had a single, monolitihic site for programming, it does not necessarily mean that model works everywhere. As someone who generally dosen't code, but has spent time in other parts of the IT industry, there's so much more to computers than coding.
So, while I don't speak for Stack Exchange as a company - its unlikely to happen. And if it does, the communities involved will be poorer for it.