You're assuming that Dia is what Atlassian wanted in this acquisition, and nothing else. We don't know what things would look like if they had continued developing Arc.
Almost every successful company has got there by grinding away on hard problems. No one launches a product and gets endless growth for free. Not to say that Arc would have definitely succeeded, but to date it's been a lot more successful than Dia.
I don't think this is a good comparison. There are many startups that have succeeded developing good products, but very few that could match the success of Apple.
It got them acquired, so certainly it worked for them this time.
> doing the hard work of building a great product
How often has that ever been a good decision?