In particular we think of Prototype, jQuery, and others as “DOM manipulation libraries”; somewhat like low-level drawing APIs.

Prototype and jQuery are more like “convenience functions” than a drawing library. If SproutCore wants to live on top of Prototype, then they are going to greatly limit the amount of stuff they can achieve if they’re expecting a drawing API.

SproutCore should live above this layer, allowing you to choose whichever drawing library you like to create custom views.

I think they’ve overestimated the power of those libraries. If you want to be able to change library providers, you’re going to have to write a proxy layer. And by the time you’ve done that, you already have enough code equal to the weight of those libraries. You might as well just write your own library as it’s not that hard. The hardest part is just the querySelector engine.