Yeah, I don't see how they could be "trapped" since this is a problem the last movie spent it's entire runtime solving, and to such a degree that the very same technology was repurposed to pull off the time heist in 'Endgame'. Getting them back really should have been that simple.
Now, if their getting out at the end there was somehow not what it appears, and they were in-fact sent to some other part of the multiverse (like say one controlled or even created as a prison by whichever version of Kang exiled the "The Conqueror" in the first place) while "our" variants of Cassie, Janet & Hank have no clue where they disappeared to, that'd be something else. That would preserve whatever they may have had planned for the characters down the road having to escape from their fate, only now it's not the Quantum Realm but some Truman Show/Matrix/They Live/Negative Zone type reality prison.
Now, if their getting out at the end there was somehow not what it appears, and they were in-fact sent to some other part of the multiverse (like say one controlled or even created as a prison by whichever version of Kang exiled the "The Conqueror" in the first place) while "our" variants of Cassie, Janet & Hank have no clue where they disappeared to, that'd be something else. That would preserve whatever they may have had planned for the characters down the road having to escape from their fate, only now it's not the Quantum Realm but some Truman Show/Matrix/They Live/Negative Zone type reality prison.