CICO is neither a religion nor a complete explanation of metabolism. It’s a super simplified model that leads to some useful practical conslusions (eg it’s easier to eat less to lose weight than exercise more, some foods will make you more fat than others relative to how full they make you feel).
It’s super accessible, you can look up the calories of whatever you eat and whatever exercise you do.
I’ve seen lots of valid complaints about CICO, but never a suggestion for an alternative that’s similarly useful with even the same magnitude of simplicity.
Note I take CICO to mean bio-available calories roughly equals base metabolic rate plus exercise out for the average person. Actual energy-balance “draw a box around a person” joules in equals joules out is unhelpfully reductionist.