This.
Fact is, Saambell's right, not everyone has max-leveled every Servant available, and thus not everyone can pick and choose whenever event Servants are a thing. But even if you can't really field any strong event Servants, you can put a few weak ones on your back line and lead with a strong non-event option. Heck, most of the guest Servants I'm bringing aren't event Servants at all - they're just really strong and have event CEs equipped.
Pretty much this. Yes, it's frustrating to newer players - or those taking a leisurely pace - that the event is gated behind such a late chapter. But the fact is, this event is hard - people fresh out of Fuyuki would be hard-pressed to clear it even with a stable of event Servants.
But Babylonia is totally worth pacing through. The story is some of the best writing in the game thus far, the fights are really good - tough, but not in the bashing-my-head-against-a-wall way a certain Camelot fight was - and it's a pretty amazing experience overall. (There's a reason, I think, that the anime chose to jump in there, rather than, say, America.) Also, storywise, this event only makes sense - as much as any of them do - if you've followed the events of Babylonia.
Anyway, this event will be back next year - and it will be lighter and easier, and you'll probably be better situated to crush it. So don't sweat it too much. Besides, Santa Altera is good, but she's not the best a welfare Servant can be. I mean, she's solid, but she's no Chloe. So don't worry if you miss her this go-round.
Part of the challenge, isn't it? You could take an absolute force of non-event Servants and non-event CEs and pretty much crush every node. Or you could take non-event Servants with event CEs for a solid advantage, with a slightly improved reward. Or event Servants with non-event CEs just to push your edge a bit, while enjoying greater rewards still. Or if you're brave, event Servants and event CEs, the combination of which is definitely less than optimal, and test your skills for maximum reward.
I actually kind of appreciate it. Unlike specific challenge levels - where you basically have to push yourself or fail completely - this event lets you scale your play level to your desired rewards. Play easy mode and enjoy leisurely farming, or play hardcore and rack up the drops.