Competing against that market would be extremely difficult. These are two well entrenched companies that their lock in makes switching from extremely difficult so you would be forced sell to new companies which means revenue early on would be very low. Also, if you don't have 100% compatibility, they will throw their hands up and switch to one of established vendors.
Yeah the Google/MS tier of integration is not the level Notion can play in. But being good friends to those integrations means they'll win against all the other productivity SAASs that get thrown around at any given company.