This fanatical comment section, either stuck in the past, moaning about Jessica who is long since out of the picture by now. No matter how many "I miss Jessica, she did way better in this or that song then member". Ofc she did, the original track was recorded with her, no wonder it sounds different. Had they never had her, I'm pretty sure songs where she was less prevalent on the track would be the favorites instead of the current ones. All the songs that are considered fan favorites has her as a main vocal. Because that was who she was to the group, the voice. She wasn't even a mediocre dancer, which is why she was more often then not in the back during choreo, and from the BTS and vlogs, she wasn't much of a personality either. To me, her one redeeming factor was that she shared the english translations with Tiffany, making her more of a front figure to the western audience more then anything else. She's out, move on. No crying over spilt milk, as the saying goes.
As for people being disappointed in them using backup vocals for the new songs.. I still take it for granted that during any Kpop live show, the audio will be playback. You watch the shows for the dancing, and you buy/pirate the album for the songs. Modern day music artists have insane schedules, short physical exercise sessions, and generally bad discipline if you compare them to the older generations where the focus during live shows was always the music, never dancing. Look up live performances from the 90s and back, see how many artists you see dancing and singing at once. We as an audience demand more then humanly possible from artists, so I say stop being spoiled little brats and be glad there's an industry producing content for you to enjoy in the first place.