If Shopify app P75 response time is that slow due to that the users are in Australia, then they should get a data center there.

In the real world, you can't just optimise for the sake of it. You need to get a business case for it. Because it all boils down to revenue vs expenses

If the P75 is bad because of Australia that means 25% of their customer base is Australian.

Should they?

You could do the maths on conversion rate increase if that latency disappeared vs the cost of spinning up a dc & running it (including the mess that is localised dbs)

I’m not sure the economics works out for most businesses (I say this as an Australian)

Probably not, because the if-statement in my post is likely false. The Australian user base is likely not high enough.