As someone not intimately familiar with the field of cryptocurrency research, I wonder if truly decentralized currencies can ever be completely secure? It seems as if one ever became important enough and there was a strongly motivated enough state actor with lots of computing resources, staving off a 50% attack forever would be infeasible, especially if the people using the cryptocurrency day to day weren't mining it actively as well (it superficially seems that more and more people need less and less powerful computers anyway as cloud based services become more popular - it's not infeasable to imagine a world where your average Joe no longer has the computing power to mine ar home because they fully use cloud swrviea). Is this a valid concern, or are there ways to address this?
The term you're looking for is finality. Some people are developing consensus algorithms that never reverse transactions but in case of an attack the chain may stop.
BTW, normal people haven't been able to profitably mine any major cryptocurrency for years.