However, I don't agree with this. For example, although, in the post-TNG time frame, DS9 depicted the Romulans cooperating with the Federation for the limited objective of defeating the Dominion, there was never any indication anywhere within canon continuity that the Romulan Empire would be absorbed by the Federation. Ditto even for the Klingon Empire. On the contrary, the only indications were that they would remain separate, even if allied, entities.
Remember what Daniels tells Archer in the season 3 episode Azati Prime. When Daniels tells Archer about the Federation he includes the Klingons. The conversation takes place in the 26th century on board the Enterprise-J, during a battle with the Sphere Builders. So we can assume that some time between the 24th century (after TNG-DS9-VOY) and the 26th century, the Klingons become official members of the Federation. The Federation of the 26th century would also include the Xindi.
On another note. If Prime Trek were to return, who would want to see the 26th century war between the Fed and the Sphere Builders? It would be epic.