Sure he was a number 1 center, but he was also on arguably the worst team in the league all those years, while Fil has been on a championship caliber team his entire career.
If the only thing that matters for a player's production is whether or not he's on a championship caliber team, then what separates the good teams from the bad teams? The fact is he was on the top of the depth chart, with the most TOI likely of their forwards, and he still didn't produce that
much. If your role is reduced, you will produce less. It's simple and it's obvious.
And yes, Filppula has never been nearly as high on the depth chart as Weiss. Just because he is the #2 center doesn't make him #2 on the depth chart. Zetterberg, Datsyuk and Franzen have always been higher up than Filppula. That places him at #4 at best. If he was here this year, he would be 5th behind Alfreddson. When Hudler was here, he was behind Hudler. When Hossa was here, Filppula wasn't even in the top 6 for most of the year. He has never been high on the depth chart, period. Not that I would agree with his placment, but that is where the management has placed him.
If you argue that place on the depth chart has nothing to do with production, you're a fool. Simple as. Weiss has played with much better linemates over his career as well. And again, has only marginally better stats to show for it.