If you don’t need any of the new features that Kodi 18 offers, then it doesn’t matter when it is released in a general update for OSMC.
But, if you really need to move to Kodi 18 on OSMC, you can do it right now via the nightly builds. There will be bugs (some major), but you will also see what has been done.
If you really need Kodi 18 now, but don’t want the bugs, then you’ll have to pitch in yourself and start fixing some of them.