FIrst, let's not get derailed with religion vs scienc talk. Not the point of this thread. I like the idea of having lategame secular counters to religion, but I don't want to get into an outside debate except as it relates to religious victory in Civ 6. Let's stay on topic, please.
"Win before they do or knock them out of the game" isn't really sufficient here. Religious victory is the only VC available in the midgame (except for conquest on smaller maps -- aka knock em out of the game). So if you don't found a religion and end up with the dominant one, you don't have any other option but to invade.
Compare that to Diplo victory in C5. Arguably the easiest victory against the AI, even though there were a ton of options to counter it. You could bribe CSes. You could use spies to coup them. Or you could bribe other civs to vote in your interests. You have other tools, like ideology and religion, that can be very handy. And yes, you can beat them or kill them. The point is, you have a ton of options.
Three of the VCs (conquest, diplo, culture) were mid-late game VCs with tough win conditions and plenty of ways to counter. The other two (science and score) were endgame conditions. If someone beats you to spaceship, they've beat you to the finish line, basically, before you could accomplish one of several win conditions.
A religious game can be won in the midgame, much earlier than any of these. More importantly, they might convert you way back in the classical era, taking away the most important weapons with which you would counter them. You even get to use those weapons on their behalf, and are encouraged to. There's a whole civ (Kongo) designed around helping your rival civ win a religious victory.
There are plenty of ways to resolve this. Spreading a religion that isn't my own civs seems very gamey, and tricky as well -- if you spawn next to the dom religion, there might not be decent access to another religion within reach. And that's a condition that might set into place before the Medieval era. I don't want the potential to have most of my counter options removed that early in the game, only to find out 2-3 eras later that -- hey! Spain's gonna win the game off the religion he gave you on t100.
There are ways to resolve this.. I don't think 'just war the guy' is as compelling as mechanics that encourage religious infighting. As a civ nears religious victory, will neighboring AIs go after him as well? That makes things more interesting, but not quite as interesting as fuller mechanics behind religious politics, schisms, heresies, and religious wars.
Again, though, don't get sidetracked by religion vs science. And yes, 'kill him or beat him' is insufficient when you should -- like the mid-lategame VCs in Civ 5 -- have plenty of valid tools to counter them.
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