There is no indication these chips would include 3D cache. 5800X3D is not low-end. As cool as it would be to get a 5600X3D or a 5950X3D, I don't see AMD cannabilizing their Zen4 sales.
I kinda like this idea better than throwing DDR4 compatibility at Zen4. Perfect cheap upgrades for folks still on early Zen systems, and wouldn’t detract from higher end Zen4 systems
They've previously confirmed they are going to produce new products for AM4. I wouldn't be surprised if they made a 5600X3D and 5950X3D. Or some variation of them. There are a lot of people still on 2000 and 3000 series processors that would benefit from those upgrades. And with inflation hitting they would be a cost effective upgrade at that.