I am currently selling to Brazilian costumers and dealing with a lot of abandoned carts.
This is a lot of friction to a simple checkout process.
In Brazil there are some checkout systems that use only one page and the majority of ecommerces have maximum of 2 screens – while on Shopify the user goes to four/five screens.
Even Shopify Blog present some better choices. I can't understand why the checkout process is so long.
Shopify noticeably have a great UX. The dashboard for the seller is gorgeous, but I don't think the same effort is made to increase conversions.
This has been discussed ad nauseum on these forums and elsewhere. There is no hard evidence that a single-page checkout generally performs better than a multi-page one in all cases. It's a lot more important to keep the overall number of mandatory inputs to a minimum, no matter if those inputs are spread across multiple pages or not.
I don't really see how Shopify could reduce the number of inputs. The only fields that are mandatory are those that are absolutely required for fulfilling the order. You could argue that Shopify could skip the shipping rate page if there is only one rate available. But that might confuse some customers, as they might wonder when they get to choose their shipping option. The way it is now it's just a single click to go to the next page.
That said, if customers want a quicker checkout, they can a) set up an account on your store (that way their address data will be pre-filled or b) use the quick-checkout buttons for PayPal etc.