I think all your reasoning is pretty weak.

They can both look close enough to Geralt if you give him the costume and makeup.

The story they have in mind likely won’t fit a time jump.

Him being a fan doesn’t really mean much.

Plenty of people can play the role you don’t need to typecast Mikkelsen into it. It’s not like cavil himself was someone who was shown to be a stern, cold and mysterious before he got the role. People thought the actor was cool, he looked cool in the costume and are used to him. His actually performance as Geralt was generally seen as good but imagine if you used that argument against him.

It reminds me of how fans wanted Bryan Cranston to be Lex Luthor because he was bald and menacing in Breaking Bad. And Cranston himself criticized the idea of not being a great way to cast for a role.