Answer

Hello,
As for the issue itself, it is indeed complicated. There is Rabbi Zeira’s stringency and the custom of the daughters of Israel, who adopted extra stringencies for themselves, and there is also an element there of an actual halakhic enactment. Sometimes it is really required by law (in the case of a zavah), and sometimes it is a stringency. It depends on which of the seven days, and on the four or five days before them. This is not the place to go into all that. So here I’ll address only the principled discussion within the framework you laid out.
I should also note that there are substantive discussions of the issue, and you can search and read. For example here: http://puah.org.il/%D7%97%D7%95%D7%9E%D7%A8%D7%AA-%D7%91%D7%A0%D7%95%D7%AA-%D7%99%D7%A9%D7%A8%D7%90%D7%9C-%D7%91%D7%96%D7%9E%D7%9F-%D7%94%D7%96%D7%94/
Now regarding the framework of your question.
First, I very much identify with your feeling that people today tend not to deal with issues on their merits out of fear of a slippery slope. The implicit assumption is that a nuanced answer, such as “the thing is permitted, but there is a danger of a slippery slope,” is dangerous, and so they prefer to move immediately to the second-order discussion (slippery slopes). I, by contrast, think the danger in the simplistic approach is much greater, because people lose trust in Jewish law and in halakhic decisors, and they don’t believe them even when they tell the truth that something is forbidden (for fear that perhaps they are forbidding it only because of slippery slopes).
Second, there is room to ask, both about a custom and about full-fledged Jewish law, whether they are relevant for our times. True, there are mechanisms for halakhic change even when the rationale no longer applies, but this is a relevant question and worth discussing. In the end, even if one reaches the conclusion that a change is needed, perhaps it is impossible because of considerations of authority, but there are cases in which it is possible. In any case, it is important to conduct the discussion in all four of its aspects: 1. On the merits (is change needed, is there a need for change). 2. On the merits (is there a halakhic possibility of making the change). 3. The second-order level (the danger of slippery slopes and consequences). 4. The question of authority. And it is no less important to keep the levels of discussion separate.
Regarding halakhic change, see my remarks here: https://mikyab.net/%D7%9B%D7%AA%D7%91%D7%99%D7%9D/%D7%9E%D7%90%D7%9E%D7%A8%D7%99%D7%9D/%D7%94%D7%90%D7%9D-%D7%99%D7%A9-%D7%A2%D7%91%D7%95%D7%93%D7%94-%D7%96%D7%A8%D7%94-%D7%A0%D7%90%D7%95%D7%A8%D7%94-%D7%A2%D7%9C-%D7%94%D7%99%D7%97%D7%A1-%D7%9C%D7%92%D7%95%D7%99%D7%99%D7%9D-%D7%95/
Third, even when asking about relevance, it is important to note that an actual change in reality carries significant weight. For example, in your case, the relationship between spouses was relevant in the past as well, and therefore precisely on this question it is harder to permit based on a change in reality. It’s not as if some substantive change has occurred here. So why didn’t they permit it in the past? True, that is not a categorical argument, since even what they forbade in the past may perhaps be something we may permit and disagree with them about. But it is an important consideration.
One can of course argue that relationships today include more aspects of affection and conversation, whereas in the past it was usually mainly about building a home in the technical sense, but I’m not sure that is really true.
Beyond that, quite a few people argue that this forced separation actually improves the relationship, since the distance creates more desire and longing for the permitted times. I don’t know whether that is true, or whether it is true for everyone, but in my view it is a significant argument. That too is a consideration that should be taken into account in the question of whether change is needed.