I suppose that from the very beginning, Satoshi knew that scalability could become an issue in the future. That's why he introduced the difficulty adjustment to ensure a constant mining rate, and perhaps he even established that rule limiting the block size to 1MB to keep the chain's weight under control down the line.

I'm not sure if it was necessarily to prevent spam, but I believe he understood that over time the chain could become too heavy, creating limitations for nodes with fewer resources.