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
That said, if you are a smaller company, you absolutely could kill Jira pretty quick.
Ticketing systems are not dumb CRUD apps, they're systems that build workflow engines. If you've never built a workflow engine, they're annoying but fine. Building an engine that can implement any special snowflake flavor of business workflow in a way that's reasonable with a reasonable UI is difficult.
And yes, you could write it for your special use case, but use cases change a lot and different groups need different use cases and the time you spend dicking around on building ticketing software that already exists is time you're not spending on shipping, and at the end of the day Jira is like $15/seat at sticker price, why are you bothering?
And that's why Jira is both terrible and still popular.