React Native could very well fall out of favor in the next 10 years, but the amount of companies using it today means there is a sufficient base to share the maintenance burden with, of which Shopify could potentially carry a large amount because of the significant overlap with the rest of their tech stack. If Google becomes disinterested in Dart, could Shopify step up and maintain it? Seems unlikely, and why on Earth would they want to, it's nowhere close to the rest of their competencies?

If for one technology you can extrapolate from today out to the next several years at least, and the other's path is effectively entirely opaque and at the whim of another party, it's a company-killer level risk to bet on the latter. Put it this way, if Google drops Dart/Flutter tomorrow, Google will be just fine as they have very little skin in the game so to speak. Technical reasons one way or another are completely overshadowed by the business risks, as the landscape is today (which will change over the years, but Shopify is making their decision today).