At this point it's natural for fans to pick sides and Reynolds embodies the character through and through. There is no other actor we can imagine who would do the character justice the way RR did.
And the sequel will happen without Tim Miller, whether it works tonally needs to the seen. Sequels perform better when the audience is given something new while retaining its familiarity, it's a tough balancing act.
If Miller was trying to explore the character in new ways and Reynolds wanted it to be the same, it might work for some while it will feel like a retread for others.
I was bummed out when Edgar Wright left Ant-Man, but Peyton Reed did a fantastic job and it was one of my favourite movies of 2015. So I hope the new director succeeds in the same way.
Though some acknowledge Miller's contribution, Umberto's and these other tweets are right that some fans are too quick to dismiss Miller's crucial role in the first film.