As you increasingly mandate things that the public thinks are optional, eventually mandates in general start to look unimportant, and eventually you get less safety seat compliance.
If there are some illnesses we can handle with without universal vaccination, then including those vaccines as mandates means you’ll eventually get less compliance for high-priority vaccines too. This is what we’ve seen play out when the public distrusts medical authorities. We live in a democratic society and (not) listening goes both ways.
How many deaths are acceptable to say we can "handle" an illness?
Public health requires over 95% vaccination. There has never been a realistic path to that other than requiring students to be vaccinated to attend school. Without that requirement, even well meaning parents forget or may not make it a priority.
It's not fair for kids and others vulnerable in society to die because certain parents are ignorant.
Every disease can be treated without a vaccine. But treatments aren't 100%, and treatments come with their own risk. Taking medicine isn't risk-free, and certainly not necessarily less risky than vaccinations. So, even if you believe it's fine to just treat the disease instead of trying to prevent it, that doesn't mean you skip out on risk. You could have more, just from the medicines alone, not even considering the effects of the disease itself.