I saw a video, it was probably mothers basement that talked about how we havent had a one real huge mecha anime thats popular domesrically aswell as abroad since gurren lagann
Code Geass was more or less released at the same time (2006-2008 vs 2007 for Gurren Lagan), but yes if the anime industry is on a decline mecha anime litterally collapsed (especially if you look at the output of Sunrise in the 80-90's vs now)
It's also why the Super Robot Taisen have been selling less and less and even Gundam which is the biggest pillar of the genre is losing relevancy in the videogame market and can't seem to find new notable success on TV since 00 also a decade ago.
They're still selling gunpla and thriving in arcade, but it wouldn't surprise me if the latter is also in decline.