How do you get around the network-effect benefits of large cultures? While more vulnerable by being larger (and a more attractive target), these cultures also develop self-defense mechanisms and an ecosystem of support.

While a custom culture may be safer from wide scale attacks, it's still more vulnerable to targeted attacks. If you're a bank and you rely on security-by-obscurity you are in greater danger than if using a tested and documented standard system.

As an aside, the potato famine was not caused by a singular condition. The law of the land up to some time before the famine prohibited the purchase of land by Catholics; only about 7% of the agricultural land was actually in the hands of the Irish. This meant the vast majority of them were serfs to English lords and had to rely on small/fragmented patches of land to feed themselves. The reason potatoes were grown was that no other crops could be grown on these small plots. In the absence of potatoes, people would have been starving much sooner.