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In non-adversarial conditions, which doesn't apply to anyone attempting to exhibit cartel behavior. Since you can't force people to propagate blocks, and the speed of light is a limiting constant, the bigger the blocks, it seems like the bigger the attack vector.
The speed of light is not the limiting constant here. The speed of electromagnetic energy traveling down a telegraph line in say
1856 is actually comparable to the speed light in a fiber optic cable today. The bandwidth is however very different.
Edit: The bandwidth of a carrier pigeon has increased conservatively by a factor of 10
10 in the last 40 years. The speed at which the carrier pigeon flies has not changed.