Then what would be the point of having TrekBBS?
The current technobabble on warp factors 1 through 9 is that they are peaks on a sawtooth curve contrasting speed against the theoretical power expenditure of warp drive (that is, the power cost that is independent of engine type; engine inefficiencies supposedly come on top of that). This is why it's better to fly at warp 7 than at warp 6.8, and why virtually nobody chooses a non-integer warp factor if their ship is rated for the next integer value.
Above warp 9, the curve doesn't feature further sawteeth. But few starships go past warp 9 in the TNG era. Once this becomes more commonplace, Trek science may well find more of these power minimum sawteeth, and label them warp 10, 11, 12, 13 and so forth. And if, say, the sawtooth that gets the label "warp 17" again seems to be the highest one in this next cluster of teeth, engineers may decide to call infinite speed "warp 18". Until one more cluster of power minima is found...
This is a great analogy to things happening in real science: we're finding patterns in how elements are put together, and every time we think we have figured out what a pattern should look like, there's still a gnawing feeling that something new will emerge and force a rethink if we just expend greater energies, synthesize heavier elements, introduce more terms into our formulae...
Of course, TOS ships weren't even doing warp 8 on a regular basis, so it's quite natural to think that they had a very fuzzy idea about the structure of the power minima series. Engine inefficiencies may have hidden the structure of the higher minima so that the TOS idea of warp 8 didn't really match the eight minimum, and TOS warp factors from 9 up were all mucked up and didn't correspond to real power minima at all. By the 24th century, this at least was remedied. But only up till warp 9, above which UFP science could only verify the absence of minima up to certain speeds (say, warp 9.91) after which engine inefficiencies again prevented empirical observations.
Timo Saloniemi