The thing about our your argument is you're asking the company to take a massive risk, based off nothing but your opinion.
Offering a discount on a shared subscription does not have any real risk. If no one only subscribed to one game currently takes advantage of it, SE is only out the cost of implementation (which should be relatively insignificant against the current profits of both games). If people do use it but it's mostly people who are already subscribed to both anyway (admittedly possible), then they're still only out a relatively small sum and they can just end the promotion. On the other hand, if it gets some people who don't currently play both to play both (and this suggestion has come up enough that there are at least some players in this position), then it is a success and SE makes a little extra money.

The worst case scenario is no one takes advantage of it (or only the wrong people do it) and it incurs a modest cost to SE. Then it's just a failed experiment and they can end it as easily as they started it.

But frankly, I don't think it would be a failiure. They've already done this once before and it was helpful. There's no reason it couldn't help a second time.