I think this is different for at least four big reasons:
1. Reagan finished his career still as evil as ever. McCain's last big act was stopping the Republican repeal of Obamacare, which was pretty huge.
2. Even when McCain did some bs stuff, he often showed that he had some form of remorse/morals that was often lacking in his party. He was accused as part of the Keating 5, but followed that up by making campaign finance reform a hallmark and working to pass McCain-Feingold and exposing the Jack Abramoff scandal. He fought in Vietnam and said some racist things about the Vietnamese, but then helped lead the effort to reestablish diplomatic relations with the Vietnamese. He fought against pork barrel spending, went bipartisan to fix the judicial nominations crisis, went bipartisan to try to fix immigration, went bipartisan to oppose torture and indefinite detention...
3. Everything that happens right now is played through the lens of Donald Trump, and McCain was basically a saint compared to Trump. Pointing out how much Trump hates McCain and how inferior he is to him in several important ways isn't just getting an "own" of Trump, it's making a valid point regarding what Trump's loss of decency does to politics.
4. This guy was shot down, captured, tortured to the point of being disabled for life, and STILL refused to be sent home early before his fellow soldiers were sent home. He spent 6 years as a POW. That's some insane stuff - when damn near every other Republican leader bent over backwards to lie and cheat in order to avoid the war, McCain didn't even avoid an extra three years as a POW when a single word would have bought him freedom. That's some crazy stuff that is obviously going to be brought up around his death.
I used to think McCain was "okay for a Republican", when he went full-right during the 2008 election I'd had enough of him. That was pretty pathetic. I don't care to lionize him in his death, I haven't posted about him on Facebook and I don't have any great positive feelings about him now. But it's okay to take someone who you don't really like, someone who did some pretty bad things, and acknowledge that they had some positive qualities and did some pretty good things too, even if they didn't outweigh the bad. I don't like McCain. But I understand why people would focus on his positive traits right now at the time of his death, especially when a lot of those are particular traits that they really wish certain politicians would at least try to emulate.