So, for tech people, Windows 7 is just as good as Windows 8 for SSDs, because they know how to enable TRIM and it isn't an issue at all. So Windows 8 represents a null upgrade, with a significant downgrade in the UI. I'm ignoring the other poor decisions (and there are many).
For non-tech people, who often look at SSDs, see the price tag, look over at the HDD and see the quintuple space for a quarter of the price... well, they don't usually even have an SSD. So any optimization on that front does nothing for them. These are the people who, no matter if it is adjustable with third-party mods, are basically stuck with the default UI of an OS, because they simply don't know enough to adjust it. And that UI is basically unusable to these people, which is the vast majority of computer users still. People in this forum are exceptions, not the rule.
Having a bad UI is a non-starter, especially if it requires third party mods to work for people. It doesn't matter how good the OS is (and, frankly, Windows 8 is nothing to write home about from a technical stand point, it has some interesting compression, but we're talking marginal benefits across the board). Imagine a video game with a UI so bad the game was unplayable. Sure, some people would keep playing and two months later there might be a community mod or a patch that makes it slightly less fucked.... but the game is dead at that point. That is Windows 8.