He is still influencing politics. That's a problem. His Presidency should be considered a warning and we should learn from it. Thus, the need to continue to discuss and debate Trump's role in American politics.
I think there's a direct relationship between how much influence he has and how his opponents are obsessed with taking him down. If they'd lay off, then his supporters would fade away, and Trump's influence would equally fade - if that's the actual goal here. The OP asked why people still believe in Trump, and the answer to that question (as it's been the same answer since 2016) is because of the people who are/were obsessed with wanting to get rid of Trump.
Let such an egregious assault on our country and democracy just fade away? We must send the message to do such a thing will not be tolerated and will lead to prosecution.
I get that, but prosecutions are supposed to take place inside courtrooms.
But my point is, the political situation in the country right now appears to be show signs of strain and growing fragility. I think it's been decades in the making, and Trump and many of his ideological cohorts knew just the right buttons to push. By continuing to go after Trump (especially after there's no any real practical need to do so in terms of America's present national interests), they risk making a martyr of the guy in the eyes of his fans.
The question is, is it really worth it? And if so, why? What message should we send, and have we really given much thought to the mindset of the audience we might direct such a message?
The country is in a rather foul mood these days, and there's a great deal of political entrenchment and intransigence - almost to the point where different factions almost seem to be coming from alternate realities. One can compare different media, conservative vs. liberal, and it's uncanny how both seem to have completely different perceptions in how they see the world around them.
I can't say I harbor any illusions about our country or our democracy, although I know historically, we've survived and sustained egregious assaults before. I don't believe that our democracy was in any real danger on January 6. I had a much bigger fear that Trump might try to co-opt the military, and possibly stage a coup - which would have been treasonous, but that's the only way a democratically elected government could be overthrown. Once it was made clear that the military had every intention to carry out their oath to the Constitution and would not interfere in the democratic process, I knew the Republic was safe. A few hundred yahoos attacking the Capitol wouldn't have changed the results of the election. Our government is not that fragile.