You can disagree with it's existance but the fan service is actually part of the story and plot. Also, it's a show pitched by Nishigori and produced by Aniplex/A-1 Pictures and other Procution Comitte members. He worked in Gurren Lagan and Evangelion and he's the director/storyboarder, just like most of it's staff is also made by ex-gainax members. So it's more of these same things that influenced his works and he wanted to do more (which is why Trigger is there with Imaishi and his staff and why the Idolmaster Cinderella girls staff that was made by ex-gainax is also there)
And no, Darling in the franxx is way more popular than those two if we're talking about reception and engagement in the west. By Japan, we'll have to wait for the products made by the Committee which will sustain the anime. Too soon for that tho.
Even looking just at Crunchyroll official slide, it's in there being one of the most popular.
Now, let's not act like it wasn't always like that. Such manga always existed along others (the first ecchi began in the 70s), just like novels and anime (including Gainax anime such as Gunbuster which popularized the boobs physics). It's been always there, the difference that now we look to all the quantity all together with the internet and you can see the different offering.
At the same time that it's obvious that such thing will happen. Different manga and light novel are adapted in anime in seasons with different content, stories and genres, that's why this always happened since those are different companies, production committees and studios doing these animes.