First for Wendy; she is talented singer and passionate about music. i think she cares more about music and will be happier under a smaller label as solo artist where she can make the type of music she likes; even if she may not chart or sell as well. Also she has always been treated unfairly by SM and she doesn't have many brand deals anyway to make it worth it to stick with SM.
Next irene; with her scandal; even if she returns people will give her hard time. i would rather she starts her own business or work in musicals and eventually get into acting.
Joy i want her to join an acting agency. With SM; she gets a lot of brand endorsements being beautiful and popular; but right now she is 24; and a good time to switch to acting where she can focus on acting and not get lost with the modeling deals.
Seulgi and Yeri are the ones who should re sign with SM. Seulgi is the idol of idols well rounded and ready since a long time for solo career. She will also benefit from brand deals as soloist under SM. Yeri is only 22 now and even if she resign for another 3 years; she will be still be 25. Yeri is also the least popular member but she is also stunning and if she continues under SM; she will get endorsement deals and acting or MCing jobs from where she can further establish her career.