OK, so first of all, this ending sucks ass, but there are a few things to give it credit for that I think are going overlooked.
For one, as has been said, sex didn't fix the loop. They indulged in it for a time and were able to have a few (maybe thousands, there's no way to know) more happy loop days together before it wasn't enough anymore and they relapsed into killing themselves, eventually drifting apart while doing so.
Secondly, the fact that they don't remember anything after the loop has finally broken ties back into the conversation they had in the movie chapter about not being able to remember anything after 500 million years (they had even already forgotten watching that same movie before in however long it had been since the first time). On that point, as the film reel continued you could see shots of him in bed, and it's subtle but that specific drawing repeats into the part of the reel that starts to burn, and I'm pretty sure it was the only one that repeated out of that whole sequence.
So it seems like after a long enough span of time, he started doing exclusively what he said he does in this chapter, waking up, checking the date and then doing nothing, essentially forever, until he eventually lost his memories of the whole affair naturally over the course of however long (assumedly she ends up in a similar semi-vegitative state as well) until one day they wake up, the loop has ended and they notice that the day has changed, which prompts them to actually get up again, even though they don't even remember what the significance of that is by now. They just know deep down that it's something they were waiting for.
So it's not that their memories magically disappeared. We can infer most of this based on what was set up in the movie chapter, and it makes sense considering that they still remember each other and their connection to some extent, even though we've already seen that the "power of love" or whatever did not actually break the loop.
Now, as for what the implications of that are when it comes to like, how they'll live the rest of their lives, I could not possibly tell you. And the fact that we didn't get anything about why the loop started or especially what caused it to end (since we could have at least speculated about the start if we saw the end) is really egregious; but to be fair, the mangaka didn't intend for it to end like this, and it is extremely difficult to wrap something up nicely under those circumstances, so I would cut them some slack.
They did at least try to set up an ending, it just wasn't the ideal one they probably wanted. It doesn't make the ending better, but you can tell that there was still something here, and they could definitely hit the homerun with the next one if they take things in stride
