As others have said, there is precedent.
and tbh, if what others believe becomes fully concrete with other advanced leveling in the future.
Then you can consider the 10th level of the expansion as breaking beyond 99, and 99 your true cap. or whatever 9th level it is.

But regardless of all that, you will have people that simply prefer or love flat founded numbers.(Regardless of obsessions, impulses, or anything else)

it's honestly a minor thing to worry about, but references would be nice.
[if it was a mmo I'd run, I'd toy with people with starting them at level '"0"', and when they finally level up.., it turns into (0).1 -since having only ten levels is small for a new one, but it would give a lot of space to work with when working a fresh game, and then stick to either 10<or 10.0> for expacs]

but that's just a untested and random idea.

the reason I even wrote that, is to point out that leveling or progression has to have a purpose. <while likely they'd have to have thought way far ahead if they wanted to do the 99, or like I said, 99 is true cap(and 100 is your breakthrough level)> but it's been clear for awhile now that 10 for each expac has been so stable for them to not steer away from it. though well see what comes after or even during Dawntrail.

[I get the want for symbolism, but sometimes not all the neat things can fit in. and then if people keep pushing, you'll just ask for a deluge of reasonable responses against it. Hopefully logical ones.]