That's some hyperbole there. Ewers came into this game 44 of 63 with 6 passing TDs. Last night, he was 14 of 16 for 185 and 2 scores before he got hurt, exiting with Texas up 14-0 and having moved them to the UTSA 20 which led to Arch's first passing score in relief. This game was never going to be a contest. Ewers was on his way to 3-4 passing scores, 250+ yards, and probably would have undressed at half-time.
I'm not saying Ewers isn't good as well, just that Manning, despite being younger, is significantly ahead in terms of his development. He is a special player whose mechanics and temperament are unsurprisingly similar to Peyton's circa 1997. Therefore, I can't see a situation where both play at UT next year. If Ewers is sporadically injured this year to where it drops his draft status and he feels he needs to play another year, then I can't see him staying at UT (or else Manning could go, but that's more unlikely).