That's not clear at all.

"Message X should be provably recent and alternatives should be practically impossible to produce."

Practical impossibility can be reframed in terms of "opportunity cost": there are limited physical resources and those should have been largely allocated to X than to Y so we can see that X sucked in all resources from any alternatives. Because if it didn't, then there is a huge uncertainty about whether remaining resources are used for alternative Y or they do not interfere with the voting process. Is it possible that X did not suck in a lot of resources while alternatives are still not possible? Then it would mean that X logically follows from whatever previous state of the system and there is no voting process needed.

Therefore: message X should be provably recent and should have employed provably big amount of resources, big enough that there are not enough resources left for any alternative Y to produce in a reasonably short time frame. Also, the message X should be always "recent" and always outcompete any alternative. Because we cannot reliably compare "old" messages: is Y an "old" one that was just delivered now, or was it produced just now after resources spent on X were released?

This logically leads us to the following: we should accept only the messages with the biggest Proof-of-Work attached, and that proof-of-work should be the greatest possible ever, so there would not be any possibility for any alternative to be produce in the short window of time. And that proof-of-work must be constantly reinforced or the value of previous consensus begins to fade quickly as the opportunity for alternatives grows.

We need, at a minimum, a majority of *active* hashrate to not "attack" the network.