So fun fact about me; I read a lot of manga!
And recently I saw a "hot take" on TikTok concerning Seraph of the End, which is a personal favorite of mine and very close to my heart, because YuuMika was one of my first "ships".
But the "hot take" was that Mika will die [permanently with no way to bring him back] by the end of the manga and Yuu will move on to be with Shinoa and have children and name one of them Mikaela. [This person specifically states that that is what Mika wanted based off what he told Yuu in the manga, 'to learn to like her back' when referring to Shinoa. (chapter 119!)]
Also in Chapter 119, Yuu stated this, referring to him and Mika 'either they try to save everyone together or they die together.'
So we are gonna ignore when Yuu basically said he'd rather die with Mika than go on living without him?
Also when Yuu was fighting Asuramaru [chapter 117], Asuramaru states '"From the start, you've only ever had eyes for Mikaela!", so based on this, I say, even if Yuu continued to live after Mika's "theorized death", I don't think he would just move on and start a family...
Based on canonical evidence ; If Mika died [permanently], Yuu will follow right behind him.
Has this person not been reading the same Manga that I have? Like seriously?
And this is not me 'defending' my ship, this is me actually understanding the characters and reading the manga, if Mika truly and definitely permanently dies, no way to bring him back, Yuu would not be able to take it. He's already lost him twice, a third time will break him, especially if there's no way to bring him back.
I am also a true believer that Yuu has only ever loved Mika [romantic love] and it has been proven time and time again.
Also Yuu would do anything to protect and keep Mika safe, so I'd say Mika dying won't happen.