Dembele as a 9 can never turn into a successful card, because he doesn't have the IQ to learn a new difficult role, especially against compact defences.
As for Lukaku, he doesn't need lots of intelligence for that. He already has an impressive physicality to win the duels, got powerful shots and lots of experience.. He is a reliable goal-scorer; and I see him very complementary with a creator like Messi (if he stays in the club).
Even in build-up, Stegen can send a direct long ball to a Lukaku's header if the opponent is playing with a high-pressing and blocking short passes.
If I go on what you are saying, and forget for a while what my picture of Lukaku is like, then I say Suarez could've played that role 10 times better than Lukaku still, do you agree?
There must be a lot of better options than Lukaku, I say Braithwaite is better, and there are hundreds like him around.