Sometimes, maybe even overtimes, an acquisition can lead to deceleration.
But not here.
Trello is a very mature, well established product, with a strong brand.
Perhaps, under Atlassian, in 4–5 years, if they don’t continue to invest, the “next Trello” will surpass them.
But products that have large traction, and established brands, and an existing, proven go-to-market channel … are not usually negatively impacted by an acquisition. Not for quite a while, at least.