Not everyone is mentally suited to carrying heavy responsibilities like saving all of humanity.
A kidnapped middleschooler certainly wouldn't be.
Honestly, I hate how most isekais gloss over the reality of what would happen if teenagers were suddenly ripped away from their sanctuaries of comfort (homes and familiar territory) and their social webs of friends and family, and were then forced to fight to not only survive but to save lives.
Teenagers are irrational and unstable (and sometimes rebellious). They're still in the phase of developing their social identities and learning to cope with responsibilities without the external help of parents or guardians or counselors.
If you dump the burdens and responsibility of people's lives into their hands, most will not perform particularly well...
The plot hasn't progressed enough for the hero to have had any sort of meaningful, satisfying character growth. The comment spoilers from the novel and the manga raws aren't exactly giving me hope, but if we wait, we might just get a decently good zero-to-hero story.
Some of the best hero stories centralize around that kind of character growth, after all.