This post is super-quickly written and mostly for reference.
Taking an idea seriously means 2 things:
1. updating your whole belief network according to the idea, including when that updates you towards weird or "out there" belief states
2. acting on your updated belief network
Compartmentalization in epistemic and instrumental rationality
Animals can suffer - duty to prevent animal suffering - stop that lion hunting that gazelle - lion suffering increase - work out how to feed lions - conclude predators and prey exist - conclude humans are just very smart predators - eating meat ok.
I'd contend that some positions are taken very seriously but what the next perceived logical step for people is varies. An animal activist might be pro the world becoming vegetarian. A non-animal activist is pro strong animal welfare laws to prevent needless suffering.
Trying to resolve "humans are just smart predators so we can eat prey animals" vs "humans have moved beyond the need to behave as predator animals" is unlikely to be resolved by suggesting any parties don't take their ideas seriously.
Otherwise you end up in all sorts of quagmires. Food security is a big problem. Ok, go start a farm then. No, I'm going to write letters to politicians. You're not taking the idea seriously.
Where can that go?
I mean, this forum talks often about existential AI risk. The climate catastrophe is known and real and here now. So... are people not taking it seriously because they're highly concerned about AI?
I have three words. Epistemic learned helplessness.
It's a terrible idea to want people to take ideas seriously.
(Of course, I'm not necessarily disagreeing with you at all, since you didn't say that it's a good idea.)