Asking and answering in case (unlikely) anyone else comes across this.
We had a few users (4 out of maybe 15) report that the default WYSIWYG editor would not activate at all in v10+ 2sxc.
We finally figured out that the client's IT dept had Chrome self-updating blocked on some of their older machines. So we verified that Chrome v56, 57, 60, and (maybe) as high as 68 all had this problem with TinyMCE activating. It would stay greyed-out when you clicked on it.
So if not obvious from above, the fix is to upgrade Chrome to the latest version. I realize this will affect very few people. Cheers!