That protagonist is trash; I could do better!

"Makoto Ito is not smooth enough." "Shu Ouma is too chuunibyou (delusional)." "Shinji Matou is too ordinary." "Sieg is useless."
"I've learned one thing from my brief experience as a viewer: the more works you watch, the more you'll discover the incompetence of the protagonist!"
"What exactly are you trying to say?!"
"That trashy protagonist? I could do better!"
"Okay, if you're so good, then you do it."
"???"
From then on, the world lost one keyboard warrior and gained one traveler who journeys between different worlds.
"Damn it (in both Chinese and Japanese), you're a genius to let a seven-year-old participate in the Holy Grail War!" The seemingly harmless six-year-old with the kelp-like hair was completely speechless.
Before he could finish complaining, his father led over a timid little girl and introduced her, "Shinji, from today onwards, she is your little sister. Her name is Sakura, Matou Sakura."