Killing Reagan does not change the political climate that allowed him to rise to power

You're both right and wrong. While it wouldn't change the circumstances that led to his election directly, he himself played a big part in CREATING those circumstances and perpetuating them to the point of the Trump cult eventually becoming all but inevitable.

Who knows? Maybe if we got rid of Reagan early enough, Bush the elder would work towards the "open borders" he argued for during the primary debate against him.

That's just one of dozens if not hundreds of issues where Bush the elder becoming potus earlier would, while not great, be much preferable to 8 years of Reagan. Hell, who knows? Might have made a more left wing candidate than Clinton the next president!

PS: everyone seems to be forgetting that, on top of his OWN bad policy choices, the country was pretty much run by the throat goat drug demonizer Nancy and his favorite astrologer towards the end where the dementia had made him incapable of doing much himself.