Actors from outside the system that want bitcoin to die, can spam it to death - AND include the blocks on their own / at their own expense, because the cost of bitcoin back in 2010 was peanuts.
The point is that by PoW consensus, every previous block is voted over by PoW by the peer miners. So if the PoW consensus is working, you might think that even if a crazy attacker makes a 10 GB block that is correct according to the bitcoin protocol, the next miner should most probably decide NOT to build upon it because he estimates this not to be the good consensus of the bitcoin state.
I think that Satoshi, who also made the *mining software*, built in this 1 MB as a MINING directive, not as a PROTOCOL specification - because bitcoin miners were also taking Satoshi's software to MINE. In fact, in those days, a node was a mining node (one CPU/one vote). So the 1MB limit was probably seen, by Satoshi, as the "no-go" for a honest miner to not build upon a crazy block. Not as a protocol specification.
He couldn't of course, protect bitcoin against any serious outside attack, because bitcoin was proof of work and the proof of work in those days was peanuts. If it would have been a PoS coin, it would have been protected (especially given the big stash that Satoshi had himself) as the attacks would be limited to insiders. Nobody attacked bitcoin at that time, because it was considered a joke by those that would now most probably curse themselves from not having done it !