Steve (our other active mod, no offence to Jivings) and I were discussing this yesterday because of the failure of the Embedded proposal, and you've actually beat me to the punch on putting up a question about expanding our scope. Not exactly this question, however. I mention this because I may still put that up later this weekend, and I don't want you to feel it was in reaction to this one.
After thinking about the proposal for the monster merge (Arduino, Robotics, us, & Embedded), I am not convinced it is a good idea, although I am not vehemently against it either. I have some pros and cons, but more cons...
Larger community means more eyes and more expertise. Although this might be mitigated by more work for them to do, I think if that were in proportion to the number of eyes, it would still be more efficient (because individual specializations will come more into play with scale).
Many of our current members are doing robotics and embedded work and/or use the Arduino, so may appreciate the more generalized community.
I'm being slightly pessimistic here because I think if a combined site were to work it could be great, but if it didn't we'd have taken something that is decent, and has been stable for a long time, and trashed it. Possibly three things, in fact.
A very high percentage of our questions are from people who are new to the pi and Stack Exchange. Because of our focus, we are a reasonably welcoming place for hobbyists, students, and "newbies" of all stripes. This is not to say we don't still expect a certain minimum effort from people, of course, or that we don't field questions from professionals and commercial/industrial users. Changing our name, however, and watering the stream down with more-or-less unrelated questions, may intimidate and confuse some people; a lot of the questions we get on a daily basis may not get asked, meaning the people who would have asked them perhaps could not find anywhere to ask. That would failing the community that is supposed to be our raison d'etre.
In this regard, it is worth contrasting our "Visits / day" stats (from the question) with the other sites; we have more than 3 times the volume of Robotics, Arduino, and Embedded put together. Of course, Embedded did not really get a fair test, but both Arduino and Robotics have been around for more than a year; Robotics has been around almost as long as us. Somebody is reading all this stuff (even if they don't/can't vote on it enough...), whereas the audience for Robotics and Arduino appears much, much smaller. We may end up driving many of those people away.
We have 3+ years worth of questions, ~8200 currently, meaning unless some significant pruning were done, more than half the questions on the new site would be from here. Since many of those were written on the presumption that they were being read on "Raspberry Pi Beta", they may appear very confusing on a much more general site. This can be mitigated a bit by adding a "raspberry pi" tag to all of them, but not much.
A decent proportion of our questions are not really embedded development questions; they are at best questions about using a particular embedded system from a consumer perspective. I'm thinking especially of Kodi oriented things, which are appropriate enough here but would be pretty out of place on "Embedded.SE". Those people could be shoe-ed over to U&L, but I don't think anyone would benefit.
It's fairly clear in the Embedded private beta that many of the people most excited about it were hoping for a site that would attract embedded developers (read: experts and professionals) from Stack Overflow to create a site where the quality is higher. This is an enviable but very lofty goal, and if it fails, I fear those people (perhaps correctly) may blame the merge. Since there will now be the Embedded mega-site (whose questions are 2/3rds about the Raspberry Pi or Arduino, presuming we both continue to generate ~10 questions a day, and Embedded + Robotics can generate 10 more), their chances of ever accomplishing that goal will be pretty much quashed.
I do not think we would have many issues defining a scope which is distinguished enough from Electronics, Programmers or Stack Overflow. After all, your site and the others have been doing that already for quite some time. Think of the suggested merge as the least common multiple of the four sites.
Something very interesting to me from the closure announcement on Embedded is this:
Looking at recent technology betas the most successful sites focus on well-defined communities such as elementary OS, CiviCRM, Vi and Vim, and Emacs users and developers.
Those are some pretty obscure topics. Although it could be that there are more vim users around than people with an interest in embedded development, you'd still think the latter a much more viable topic for an entire Q&A site. So what's the deal here?
Notice the first thing on that list is for Elementary OS, a "Linux distribution based on Ubuntu". But there's already a full fledged Unix & Linux and Ask Ubuntu! How is this necessary or feasible?
I think it has to do with the specificity. I had not thought about it before, but it helps to explain why we have remained stable;1 people use this site because they are Raspberry Pi users. It is, as you say, what distinguishes us. I do think we could open our scope up somewhat to include some similar hardware (that's another question...) but going with the "lowest common multiple" may in fact result in something which pleases no one, unfortunately.
Put another way, there's an element of psychology/user perspective at play. It seems to me this was no small part of the original impetus for the Embedded site: Some people felt they were getting a short stick on S.O., U&L, etc., and wanted their own place separate from that. It is easy to say all these things fit together conceptually and I agree, but my experience as moderator tells me that many people, particularly if they are new to the whole SE system and milieu, won't be comfortable, making it harder (not easier) to attract users with a broader scope.
1. Full disclosure: I was not part of creating this site and joined when it was already a year old. If I had noticed it in Area 51, I would have gone, "Nah, that's silly -- too specific. Now a general Embedded site, that would be a good idea...". Occasionally I'm completely wrong.
I'm most sceptical about Robotics.SE.
I think the best idea actually is a "Robotics and Embedded"; it seems to me these two topics fit very well together. After all, a robot is an embedded system. Stats wise, the problem on Robotics is just low volume; they have a noticeably better "asked and answered" ratio than us or Arduino.