Trello covers a market that was not really served by any Atlassian products (or rather, market segments that weren't well treated).

Trello has a huge active following, and a lot of users who would otherwise not know Atlassian products.

Getting any sort of proper cross-selling going on would be great for their ecosystem long-term. People might start at Trello, but go to Atlassian products as their team scales and need other coordination tools.

I don't think you spend $425 million to kill a "competitor", especially when that competitor has a lot of paying customers anyways.