As the biggest Mittelstadt critic on the Sabres board, I'll even admit he's had a solid season.
I think, given where his contract is at, Buffalo should look at all options for him.
Keeping him this summer is fine
Moving him to address other needs on the roster is also fine. His upside isn't that high (probably a 60 pt forward in a career year and float between 40-50 points depending on usage/PP time, etc). Similar to a Josh Bailey.
The only mistake Buffalo can make with him this summer is giving him a long term contract. Given the absolute metric ton of draft capital spent on forwards, they shouldn't be giving any middle six wingers long term deals unless they are irreplaceable. Mittelstadt isn't.
If a team wants him for a D-man or goalie, fine moving him along. Not going to sell him off for picks (well, yet. Next summer would be another story)