You're still totally neutralized until you reach that backup...
EDIT: Can that be done to a fighter's maneuvers? His bonus feats?
except the wizard is not.. any spells he has in his head prepared stay prepared until they swap them out...
so if they have the right spells set up, getting their copy of the spell book is not all that hard.
never mind the fact they still have cantrips (which = basic attacks)
lets also look at bards they have "spells known", Warlocks, Sorcs, and Clerics... no need of a spellbook and they can cast ritual spells.
heck the warlock can just "summon" their spellbook if they are warlocks of the tome
want to see a useful spell in combat everyone underestimates? unseen servant, 1hr duration, ritual spell.... find familiar tons of in and out of combat utility, again ritual
like I said play a champion with 3 feats and play it without them... night and day.
not saying the fighter sucks without feats, but they are just not as good as all the other classes, even the ranger (who is almost as good in combat and has tons of out of combat utility) until they become a storm of arrows, while the barbarian suddenly is a better tank to mitigate damage and hold back the bad guys.
fighter has 4 attacks a round, very limited ways to use reactions and bonus actions (unless using 2 weapons) while every other class has a way to use those basic mechanics to do things cooked into the class.
like I said, don't like feats.. let only fighters take some from a small list you approve.. compromise