I don't agree that completing endgame content is "impossible". I think a better assessment is that it's very inconvenient and the gameplay experience is poor quality.

For example, while I have had time wasted and lags and disconnects during hunt trains and Extreme trials, the fact is I've done many hunt trains and farmed over 100 Doomtrain totems despite these issues.

While crafting can be affected, it's not "impossible" since people can travel to Materia and do it and the same is true for Deep Dungeons.

It's very unfortunate if someone is disconnected when about to clear Savage or Ultimate, and it's a poor quality experience, but it's not "impossible" to clear them since it will probably be cleared at some point in the future after that.