If it's still 9 months away, there's no real reason to announce it publicly before the Christmas season. The fact that the original Switch's sales are flagging is not a reason to announce, since it's not launching in time for the holidays. Its announcement isn't going to spurr Switch 1 sales.

When it's announces will be entirely deoendent on when retailers need to know launch details. Once it's outside of Nintendo, they'll have to announce things publicly or risk losing control over the narrative.