The values set for level sync are specifically chosen to take into account the fact that people have mules, don't always log off, and aren't all on one character.
As far as the actual number of people playing, very few have 3 characters running. In fact, at any given time, about
75% of all characters logged in are from unique IPs:
https://na.nasomi.com/status/Like was already mentioned, nas likes the values set currently, and they will not be changed. If anything, there is less reason now than there ever has been - as when we have 900, it is still 10 sync. Previously, when we were hitting, say, 700, it was more difficult to form parties comparatively (but still not hard). As the server continues to grow, you will see sync ranges larger than 10 less and less often. Eventually, the minimum online at any given time may be 600 (it's already grown from ~100 to ~400 in the past year!).

(
http://nabutso.com:4438/FFXI/index.php)
Nas tends to grow after summer ends, and then shrinks after the winter - but instead we've had growth all the way from winter to the beginning of summer. If this keeps up, and population trends stay the same, we'll be dipping to 600 at the lowest in, with maybe 2000 peaks by the end of this year.
So.. don't worry about 10 sync. You'll have a lot more people to play with in the future. And the sync will still be 10 (not only does he like his system but he also said it isn't going any lower than 10).