I also kind of wish Rust had a two-tier system where you could opt into (or out of) the full thing.

The lighter version he describes would make some assumptions that reduce not just the mental overhead but importantly the _syntax_ overhead. You cant have Arc<dyn Animal> if what you describe is an Animal and the readability is infinitely more important than performance. But if you could create a system where in the lighter Swift/C#-like syntax things are automatically cloned/boxed while in the heavier Rust they aren’t, then maybe you could use Rust everywhere without a syntax tax.