Even though Yeri joined Red velvet only after 6 months of their debut; so they have been 5 for most of the time. But Yeri was much younger than them; even Joy was 18/19 while Yeri was just 16 when she joined Red velvet. And the first songs they recorded and performed was "Automatic" which luvies will know is a somewhat sensual song and not appropriate for a 16 year old.
Also SM had Park Jungyeon who is 97 liner so she could have easily fit in the group. if you don't know she was in bts love yourself hightlight reel with Suga. She is also a good singer she uploads her singing clips on her insta. if SM wanted someone to bring youthful energy to the group then Jungyeon would have been a better choice.
Also as it's known Yeri trained with SM rookies girls not with rest of Red velvet. SM should have debuted them later in 2018. Yeri; Hina; koeun; Herin and Lami. The SM rookies who trained together and Yeri would have been their leader.