Be content with people's diffent beliefs, provided they are not harming others.
As for the costs/your partners spending that all depends on how you share your finances. What works for us is a fixed weekly allowance thing for personal expenses, everything else is joint. If the cost is significant, then there's no reason why you can't FIRE earlier than her.
And for the conversation, just politely leave the room when it comes up. If they press you on it, feel free to be honest. There's nothing wrong with saying "I know this is important to you, I love you, but in my eyes this is bullshit".
If you can't be content and want to change their beliefs, you probably have to understand what they are using these beliefs to fulfill. Lack of control, a topic to bond together, general disgust with how clinical the world has become, etc etc. Then steer them towards a complementary habit that's not "harmful". Gardening, volunteering planting trees, hiking, etc etc.
And finally it may just be a phase and they'll work their way out of it in time.