Rationally hashing things out is done in even the most illiberal contexts however. It’s not like rational debate is somehow absent from Nazi politics, corporation governance, and mafioso gatherings. If you have shared values, there’s usually plenty of incentive to rationally figure out how to further them. When that breaks down, it’s usually because you don’t genuinely have shared values — you are entering the realm of the political, where you start bargaining for advantage and forming friends and enemies.
The same issue comes up in terms of phrasing delivery ‘in a liberal sense.’ For millennia near eastern tyrants would declare how their reign delivered a bounty of good and freedom to all people in their land, restoring x y and z. Why? Propaganda. Every political order is usually incentivized to propagandize and secure consent and support from as much of the governed as possible. That means political speech is usually calibrated to indicate friendship towards a large mass of people and hostility towards only a hated, wicked minority of intolerables.
The result is that political speech always over promises on every axis, and it almost never openly affirms drawbacks. You mention some right wing examples; left wing examples are immigration (never, ever bad, never ever has a cost) diversity (never, ever bad, never ever has a cost) urbanization (never ever bad, never ever has a cost). If there are any costs, moreover, only an evil person would consider them a problem, and besides such costs shouldn’t ideally happen and something that shouldn’t ideally happen isn’t technically a real cost. Such is the dynamic.
I’d agree very much with your last comments about how debates often involve hidden value conflicts. My point is rather that technical rationality is usually never really at issue for either side in such discussions, which are proxies for contesting systems of social dominance—a fundamentally irrational power struggle. You can make purely technical policy arguments in a void, but if group A believes your ‘rational’ policy is being invoked to weaken and fuck them over politically, they won’t accept it. And this is often stated explicitly — climate change, for example, is perceived as another mechanism to shift mass western social power to a confluence of capital elites and their third-world tributaries. Tesla driving urbanites and the global capital flows they depend upon. Combined with promises about how really everybody is going to benefit and nobody is genuinely going to get screwed.. or at most, only a hateful minority will get screwed.