If you would factory-reset your device right now, it would reset to the version of Play Services that came with the installed device firmware, but upon startup the services framework would likely fetch information that it is outdated and won't continue until you have upgraded it.
In this state you could probably use your device and sideload apps, but none of the Google Mobile Services (Play Store, Gmail, Maps, YouTube,...) and 3rd party apps which require Google APIs will work