Frans is a Selke level defender... I'm not joking there. One of the very best two way centers in the game and is extremely underrated. I'd take Frans over Soda every single day of the week, I'd actually say Frans would be the ideal #2C here right now.
I said replaceable... not necessarily in free agency. Soda will be a #2 or #3... For those spots the Avs may have Duchene, Jost, Grigo, Compher, Colborne (I don't like that option at all, but it is one), and potentially Beaudin. Duchene would obviously be the #2 pick, but #3 could be any of them. Jost easily has more potential and could be better year 1. Compher could round into being a very effective #3 by next season.
Soda doesn't have much foot speed to begin with, if that drops off over the next couple years, he won't be able to keep up. I don't worry about most of his game or injuries... I worry about pacing. I like Soda, but if I'm choosing between Soda and Picks, I choose Picks.
There will be better players than Grigo exposed (off the top of my head, look at Tampa). Hell, there might be better players on the Avs exposed (Soda).
Soda may be better now, but as you yourself have pointed out, there are reasons to be cautious. Grigorenko is 22, and 6'3", and size helps even if hes not physical. Hes also getting better at face-offs, and proving to have a hell of a nose around the net.
I'd take him long term over Soda right now looking at the future.