Totally agree. In addition to the afterlife issue, one of my main take-aways from the Athrabeth is simply that Elves and humans see the world in very, very, very different ways. Their ways of being in the world are just so different, so their priorities, reasoning, and decision-making processes presumably diverge as well.We need to educate the audience as to how and why this isn't just a 'two people from different countries' situation. The 'sundered fates for all eternity (or at least until the end of Arda)' issue might not be obvious until it's pointed out. People are used to thinking of the mortal of a mortal/immortal pair dying first; they're not necessarily used to considering a segregated afterlife being an issue.
Maybe that's part of why Finrod says that if Elves and humans every marry, "it shall be for some high purpose of Doom." Setting aside the spiritual aspect, maybe sharing a great Doom takes the couple so set apart from ordinary life and society that they have more in common with each other than either has with their own people. I think you can see this in both Beren/Luthien and Tuor/Idril. Before either couple get together, Beren and Tuor spend years wandering alone. They're completely cut off from their own people and the rest of the world. Luthien and Idril are both still part of their respective societies, but they both end up rebelling against their kings to some degree. It's like the couples come together only when (or while) their ties to everything else are cut, leaving each of them the only person the other can relate to on some deep level.
(Not sure if that last part made a ton of sense...sorry!)