Would have been possible, but it would lack the great thrill of the crisis. Non-MP civ-like games are usually decided pretty early on, and then it's just a question of whether you want to click a few thousand times to play it out or not. You don't need to capture all capitals to know that you've won when you already have 1/2 of the map. There is/was a need to shake things up. And the idea that after such a "traumatic event", you continue with something new, is very appealing to me. As is having contemporary civs (I don't care fore the immortal leaders, I would gladly get rid of them).In my opinion it would have been much better to put a system of evolution of Culture with the civilization that gradually develops traits (either positive or even negative) with the possibility that these traits open up the possibility of particular buildings, particular types of units with unique characteristics of the civilization , particular forms of government etc... a bit like religion was in Civ 5 (or doing something better) instead of being forced to go from Egypt to Mongolia just because you have 3 Horses
So, for me, and this is surely very personal, just stacking some bonuses and playing on is a more boring variant. I know it is not everyone's cup of tea to create these kind of dramatic narratives with foreseeable outcomes. People and play styles differ. But I'm sure we can all get behind: rather a dramatic and effective crisis than dark ages with are actually not a penalty but make you stronger...
I've heard that the crises of Stellaris were a great inspiration for this, but that's the one PDX GS that I haven't actually played. I would be happy to hear some Stellaris players report if they actually are that good as a disruptive and rewarding element.