See, I think that's gonna be a problem. I think you're right, when casuals see Davis as Waller, or Robbie as Harley or whoever, they'll assume the new movies are just a continuation of the DCEU. Who wants that? Look at how their films performed the past year or two; they've done even worse than they deserved because the brand damage runs so deep. People are actively avoiding DC, how is it gonna help when the same faces are showing up? Instead of new actors to help herald the idea that this is a fresh start, Gunn is muddying the waters by keeping some of these actors and the last thing WB needs is audiences connecting the new stuff to the old. Even saying something like "Oh, they recast Superman" isn't gonna help as much as it should when Davis is there, showing people that despite the recasting it's still the same DC, and therefore not worth the ticket price.
Hardcore fans like us know the difference. We know Gunn is a new hand on the wheel, we know Legacy starts a whole new canon. Plenty of casuals don't, they'll just see actors connected to movies they didn't like, and if the actors are the same then what else is, and what reason do they have to give the new project a shot even if some other roles were recast? The presence of the old guard might tell casuals that it's still the same mess they learned to avoid.
I don't think this is a major problem or anything, keeping these actors isn't gonna kill the new DCU before it starts, but it's another hurdle when WB needs to smooth things out as much as possible.
Bottom line; the DCEU was so bad even keeping the good bits just isn't worth the association.