This is a good point. It looks like google treats subdomains as internal links to the main domain. No idea if they treat myshopify.com this way. FWIW, it looks

This is a good point. It looks like google treats subdomains as internal links to the main domain. No idea if they treat myshopify.com this way. FWIW, it looks like a 301 to a new domain should transfer the seo juice. If the site is valuable, a new domain should stand up pretty well. It also decamps from myshopify. This all seems more valuable to the store owner, but I get why people would want to avoid all those changes and just try to figure out the thing "Apple is making them do."