There is no way at all to implement something like multiplayer into a game like this without having to make compromises for it to fit.
There's no way to do anything in this world without having to make compromises. There is absolutely nothing about Eternity or any other game in existence, that is not, on some level, a compromise.
So this "Well, it's a compromise and thus bad!" approach to this makes literally no sense. It gets rolled out like clockwork, but it never has made sense as a free-standing, context-less criticism.
The real question is, how do you lose, how much do you gain, when you implement MP? If we look at the Infinity Engine games, they had multiplayer and lost approximately nothing. I mean, does anyone think BG1/2 were "compromised" in a negative sense by their multiplayer? If so, please explain how, in detail. With a game like this, multiplayer is not a gigantic technical obstacle, and we also need to remain sane and remember that the team who development multiplayer, would not be the content team.
I've bolded that so no-one gets confused and starts making wild claims about how implementing multiplayer would prevent X areas or Y bosses being implemented in an expansion, or starts claiming that MP would "impinge upon" content in general. It would not. The team developing multiplayer would largely be programmers, not content developers or artists.
Now, the nature of Eternity means that MP would have to be kind of limited - you'd need one player to be the "main", and the other player wouldn't be able to interact the same way as them with the NPCs and so on. They'd largely be controlling, say, half the party in combat and dungeon-type situations. Anything more than that would have needed development from the ground up, like D:OS.
So I think we have a situation where we have a relatively low cost to acquiring MP, but also a relatively low (at least at first glance) benefit from acquiring MP. So one could go either way on it.