Honestly, while it wouldn't totally surprise me to have Republicans sink to an even lower low... I'm just not sure that there's a large contingent of national voters for who "actually slavery was pretty great for black people" is an important issues or a winning message.
I feel like it's ultimately suburban voters who tip the scale in a general election, and defending slavery isn't probably the main issue on their mind. And for the GOP primary, there's no chance that he pulls anything meaningful away from the Trump cult with this shit. It just seems so stupid.