It shouldn't. Edit your life. Edit your Slack channels.
When someone leaves your beloved Slack channel, do you jump all over them? I've even been immediately invited back into a Slack channel (mostly jokingly) with comments about how I will never be able to leave.
We should make it ok for someone to leave the channel. It doesn't mean they don't care about what happens there. It's not personal. You can still associate with this person, even though they don't value what happens in that channel as highly as you. Don't be insulted when they leave. It's just a Slack channel. And who knows, they might be back -- of their own accord.
Have you ever wanted to leave a channel but were intimidated to leave the "uh oh, Jake left the channel and doesn't care any more" notice?:
This should not be intimidating. It should be ok to come and go as you choose. That's how it is in real life. When the meeting's over, you leave. When you've written your letter, you press send. When the party's over, you go home. Why then, does a Slack channel, arguably less important than all of those things in most respects, have this great pull on us.
We can't give our attention to everything all the time. Increased Slack channels are an opportunity for increased interruptions (I'm looking at you,
@here notifications) which are an opportunity for decreased productivity. If someone needs you, they'll invite you to a channel or DM you. Don't worry, you're not missing out. And people will be ok with you leaving the channel. Lol, I hope. We should be, anyway.
How do you convince yourself and others that it's ok for you to leave the channel?