As has been pointed out, you're overstating how much less powerful Highway is in a kingdom with no +Buys. It's not at all uncommon to buy a Highway with $5, even when you lack +Buy. Unless your deck is a wreck, it's usually going to be better than a Silver, and it's not all that uncommon (though decidedly less common) to buy a Silver with $5.
In the majority of games, there are key $5 cards you need to get as early as possible. But it's not always the case that you want to rush them, either. After a single Witch, maybe two, buying more Witches actually harms your deck. A Highway, on the other hand, is a welcome consolation prize for not hitting $6.
Conspirator is, at its worst, less worthwhile at its cost, because Silver is almost always better than an unactivated Conspirator, which costs more. But the percentage of boards with no +Action cards is SO tiny. I thought I remembered seeing some math on the subject, and I think it's less than 1% of all random boards. That might be a phantom memory, but I am pretty sure that's correct by a safe margin anyhow. In contrast, boards without +Buy are quite common by design.
So yeah, on many boards, this is a card you would never ever buy, not even as a consolation prize, because it would actively harm your deck. And that's something of a problem. You could make it really powerful IF activated to try to counterbalance that, but rather than having a balancing effect, it would actually just make it overpowered on some boards, underpowered on the rest, and never precisely right. Which is the reason why there's an unofficial "no -1 buy" rule in the first place. I kind of think that's where this card falls.
