On the other hand, I find the integration between Atlassian tools to be pretty barebones. It feels very clear to me that Atlassian tools are really developed separately, with a separate jira task to integrate specific parts. There's no "coherency" to them.

You are right that it's difficult to replace the entire Atlassian suite. The thing about Atlassian is that when there's if there's a box to check on a feature list, they've made sure to check it. If you go around the office asking everyone what features they want, Atlassian is going to check all those boxes. That's pretty hard to compete with.