2. If people think they can get an abnormally high return, they will invest more than otherwise.
3. Whatever other money would've got invested would've gone wherever it could've gotten the highest returns, which is unlikely to have the same ratio as US AI investments - the big tech companies did share repurchases for a decade because they didn't have any more R&D to invest in (according to their shareholders).
So while it's unlikely the US would've had $0 investment if not for AI, it's probably even less likely we would've had just as much investment.