They're also used both in startups and in established firms, which makes me feel confident about their growth.
To me that seems like a sign they are ready for new competition, but I really wonder what the mindset is like internally once an org makes that kind of shift.
The company I work for has both JIRA and Confluence. It took a lot of work from several team members to get it to the current state where most people are happy about how it works, but the output is very highly valued by higher-ups.
It also has a growing ecosystem and good integration.
They've also made some underrated acquisitions, like the JITSI team (Blue Jimp), which recently made ENSO and is working on Hipchat
The customizations I've seen at rather big companies have required multiple devs, many months, and the end result has had some rough edges.
From what I've seen it is also non-trivial to scale their products to say orgs with 500+ active users, granted my experience here is a few years back.
That's the only reason to use Hipchat today, and the Slack experience is an order of magnitude better.
I think Slack put some pressure on the Hipchat team but there is plenty of stuff that is highly problematic for how Slack is set up at the moment. In particular even for small teams it can become incredibly hard to see what's happening and the integrations either spam too much or are too useless once it becomes a bit more crowded in a room.
When you go public, you have a legal obligation to your shareholders to make more money next quarter until the end of time. I'm questioning how big they can get with their current products... when they start trying to get into different industries that are already using Oracle, Salesforce, SAP, etc, how will they compete?
I don't see that there is a limit to how far they can grow. They are expanding their user base, they are constantly making or buying new products into their portfolio and they know how to sell and support it.
Jira's a workhorse, not a consumer device. If Atlassian went with "apple-like perceived simplicity" their product would probably be terrible for most of their users (enterprise customers) who want customizability and integrations over prescribed use cases.