Dach had a much higher ceiling than Romanov. This is not fanboyism, it's realism. Please try realism, it's useful.
Nobody dreamed of Romanov getting drafted in the top 10, never mind 3OA.
In his D+1, Dach was the 5th best F for Chicago in the playoffs, with 6 points in 9 playoff games (better than Demidov, who is even a few months older!). In Romanov's D+3, he was healthy scratched in 18 of the Habs' 22 playoff games, behind really low performance players like Kulak, Gustafsson, and Merrill (!)
In Romanov's D+4. he was outproduced by Arber Xhekaj at the same age, and similar production to Jayden Struble. In Dach's D+4, he produced like a 2C and the consensus was that he could possibly challenge Suzuki for 1C soon. I invite you to go back and read the posts on this board as well as articles by multiple NHL analysts outside of Montreal.
Dach's knee injuries do not erase his ceiling at the time of the trade.