I am not local to Belfast or Paris, but I don't see how any new route can be judged for the long-term on bookings during covid exclusion periods. Even if routes like this need to be suspended for a while, I hope that carriers will give them a fair chance once 'normal' travel resumes and regular visitor activities again become possible in destination cities.
I only travel over from Manchester, but even I am waiting for covid-circus rules to recede before visiting Belfast again. Last time I looked, I needed a covid test even to go there. And the rules are like quicksand, changing from week-to-week across the home nations. Hoping for more travel stability after Easter.