I think any company with some motivated developers and a budget for h/w or cloud could rebuild a good enough JIRA in a month. I don't see how Atlassian survives long term

Because you see the IC side of the Atlassian toolkit. The management side is much more expansive and this starts mattering when you are coordinating larger projects.

That said, if you are a smaller company, you absolutely could kill Jira pretty quick.

Jira isn't the product, it's the development platform that builds the product (which is a codified version of all the bad business decisions your company has ever made).