Seamicro was well positioned and had some neat tech, I think they would have been moderately successful in the data center had AMD not bought them.
Maybe. The fabric was interesting, but it's a bit meh. Don't forget that the commodity 1U servers have full ILM, and properly implemented WOL so you can power them up and down remotely to scale demand quite cleanly. While you don't have the same degree of fine grained control, it's a decent enough approximation.
I mean, it's possible. Back when seamicro was a thing, I had some money for a moderate amount of compute (not as much as a whole seamicro box), but I spent a while looking and checking price, performance and so on before making the purchase.
The thing that surprised me was how awful the "special" solutions were compared to the COTS ones. I was looking at full 5 year cost since I had to pay up front for 5 years of electricity and rack fees---it came from a grant, so it had to be included. I think the likes of SuperMicro have been putting in a lot of good effort but just don't have the hype to back it up. "we build a nice server" just doesn't sound as good for marketing :)
As for the Atom processors, I've not seen anything where they kick ass on performance/watt. Perhaps you have some links.
Well, as always of course it depends.
For workloads with weaker in-thread parallelism, the Intel Core derived processors do better than anything if you can keep the FPU busy, and the expensive OoO unit is good at doing that on a lot of workloads.
If it's a bit more static, then having a weaker speculative and out of order unit is OK because you can statically schedule things. Push that far enough and you can get GPUS, where the silly little weak AMD APUs absloloutely trash everything including the top i7s on certain workloads.
Backing up, though, fast single threaded processors are popular because they're flexible and will run any given task at a decent efficiency and a decent speed.
Going for super specialised systems can give a big win if you always fit within the envelope, but big losses if you move outside.
TL;DR: I've seen lots of special solutions come and go over the years, but the general purpose CPU ones are preenially popular because they are cheap and do everything reasonably well.
PErhaps seamicro could have carved a niche successfully, but it would be very hard to do and harder still to keep, I think.