I am surprised that you missed to note the fact, that our brain evolves as well. E.g. the brain size gets larger, and that there is a discrepant trend of preferance of slim women which makes natural births more complicated (bigger heads need to pass slimmer hips). But medicine has evolved accordingly and cesarian sections are part of the solution, right.
Good that you brought up the question of single or multiple future human (sub) species. In fact I wrote about this very topic as well here.
In addition I think we need to also include the AIs into consideration, since for sure we will have to figure out how we will arrange with them.
And lastely I am convinced that we will upload our minds into computer environments to finally overcome our bodies for good.