I can't speak for your parent but I'm aware of Zawinski's Law and I could see that's what the comment was about, but like your parent it's not at all clear to me why, it's a non-sequitur - this is giving Rust a convenient safe way to up cast, that's not anywhere in the ballpark of the sort of "expand to do everything" that Jamie was describing as inevitable.

If you say "Ooh, the new kitten is learning how the cat flap works" and I respond "Those who cannot remember the past are condemned to repeat it" and then you respond with confusion that's because my quoting of Santayana is a non-sequitur. I might even agree with his sentiment about the importance of history, but the kitten learning how to use a cat flap isn't anywhere close to what Santayana was talking about so...