When to becomes obvious he is taking the party down then and only then will impeachment have any chance of happening
I can imagine a scenario where he goes on tv and publicly resigns, blaming Republican intransigence and white guys, before any movement to impeach him goes full steam.
Besides, impeaching means nothing unless he is also voted to being removed from office. All it means is that he can’t run ever again.
Bill Clinton seems to be doing well, despite having been impeached.