The problem is that Merckx won every sort of race. Hilly classics, cobbled classics, sprint classics, monuments, sprint stages, climbing stages, 1 week races, Grand Tours... and he won all of them, a lot and in different ways, solo, sprinting, didn't matter. It didn't matter where he was at the start, he was always the man to beat. Be it the Tour de France or Paris-Roubaix. He didn't simply win the GT's, he also still has the absolute record in stage wins in the Tour and all combined i think. More than any sprinter (Cav comes closest in the Tour).
I think this day and age, you'd have to take into account that it isn't possible anymore to do everything that well, but within a certain branch. Let's say if Gilbert could have done 7 years like he did in 2011, then i think you could say in the current age, that he was "a" new Merckx. Had Contador won 10+ GT's with dozens of stagewins, that he could have been "a" new Merckx. If van der Poel or van Aert or Pogacar can do what they did last year and so far this year, for +/-7 years running.