the side characters may be side characters but they're not 2 dimensional and some have a lot of lore to them that people were deeply interested in much like the main characters so it feels like a main character died even if they are just a smaller part to a bigger story
yeah i get thinking it's stupid if someone stops reading if a side character died but if they were mainly invested in their story then its totally fair especially if they were mainly reading the story for that character
fujimoto is an amazing author and artist that made us get insanely invested with even the side characters so getting upset over people liking them is almost insulting to the work fujimoto put into them