I worked at Shopify - Pay is all about getting the Shopify brand into consumer minds. They had a huge problem where retailers used their platform but there was very little stickiness, the customer relationship was owned by the retailer. With Pay Shopify used it's existing install base to collect customer info and intermediate their relationship with merchants, making it harder to move a shop off Shopify.

Of course they need to tell a story about increased revenue to the retailers, because otherwise why would they voluntarily give Shopify control over their relationship with their customers?

That's pretty funny. When I checkout I don't care if it's Stripe, Shopify, Square, or the business owner's nephew processing my transaction. Why in the world would I make shopping decisions based on who the payment processor is? It's like choosing stores based on which point of sale terminals they bought.

It reduces friction for purchases.

If I am on my phone, I am more likely to complete a transaction if I can just hit the Shopify Pay button. If I need to complete some form that looks like it is from the early 2000s, I might wait until I am in front of a computer, by which point I may lose interest. (There is a lot of middle ground between these two extremes though.)

Similarly, I am more likely (when there are multiple alternatives) to revisit a merchant that uses Shop Pay because of the reduced friction.

Yes, you're right - please see my reply to a sibling comment.

(I don't doubt the 75-85% figure, rather the $1T one.)

I agree, the $1T figure seems a bit fantastical.