One thing is for sure. The mangakas can be bitterly ironic, but they're not nihilists. If Tomoko ends up alone, she'll have moved forward; she's not going back to the starting point.
I think that a sad ending would, most likely, be Tomoko back with only Koto and Yuu and (of course) Kii.
The fact is that even under the best circumstances, most friendships vanish with the end of high school, with the former friends sometimes becoming new sorts of friends. The question is whether Tomoko actually loses her friends, as in a chain reaction from telling Nemo that hers was a pretty pink color, or Asuka mysteriously proving to be not so great, or Yuri's neuroses finally driving her back further into a hole, etc.
Personally, I don't think it will end up badly.