If the captain knew there was something wrong that would have risked a larger failure in the air, wouldn't NOT deplaning have potentially cost you and Delta even more time and money, or worse?

What many of us are saying is that in the absence of knowing all the facts, sometimes you need to have some trust in the people who are actually in a position to know more than you. This applies to both the FA and the pax.

This is separate from the discussion of whether the FA or captain acted appropriately/professionally.

We will probably never know if the captain was "right" to deplane, but based on everything you've said and repeated here, I very much think the FA was unprofessional in her actions, regardless of whether they were justified.