Re: Why the Xindi never appear in TOS / TNG / DS9 / VOY
...Although the only way we can really tell is because VOY "Threshold" tells us that Warp 10 now equals infinite speed, whereas in TOS it clearly did not. The rest of the TNG era ships could simply be unable to go faster than warp 9-point-something on the TOS scale for various reasons.
RAAARGHBLARGH! Threshold isn't canon!
I had a friend once, but the wheels fell off. Sad, very sad.