Is it impossible for them to return?
Rashi: It is difficult for them to separate from her and repent.
Malbim: This corresponds to "Ki Shachah El Maves Beisah" (19) One who comes to her house will not return from there; he will die.
Why will they not attain Orchos Chayim?
Malbim #1: This refers to "v'El Refa'im Magelosah" (18). It is not like one who goes on a roundabout path in order to return to the straight path.
Malbim #2: [Those who go to a Zonah's house will not repent, and] even if they repent, they will not attain Orchos Chayim, for the mindset of heresy will cling to them and sour their Sechel, like sourdough ferments a dough. Chazal said that one who separates from heresy dies immediately (Avodah Zarah 17a), for if he will remain alive, he will return to his folly.