My personal perspective on the matter is that:
in most cases it’s perfectly legal for people to take the modules and make clones, as long as they assolve all the formalities specified by the license. Not all people do it, or do it properly, but that’s another topic.
It should be noted that, among the original goals for open sourcing the designs, was to enable people to fix things themselves or make modifications, if they had the skill to, and to enable people to learn from the code and the schematics. In fact that has happened on many occasions, and there has been many examples of virtuos circles deriving from the open source license, as @baleen already pointed out below. There’s derivative modules that are really just clones and there’s many uses of the open sourced code/schamatics that took the concept further in some or many ways. Enabling people to mass-produce clones was of course not one of the goals. But again, they can legally do so.
For modules that are no longer being produced, having people still make them is good! That’s one of the great things about open source indeed. I don’t see how anybody would want to use Peaks over Stages, but that’s just me. Which is to say: you shouldn’t care, really! One of the beautiful things about modulars is that they are a very personal thing and there’s no right or wrong. Which btw. makes designing modules a big challenge! If somebody thinks Peaks is inspiring and keeps you in the flow, or just does what you need it do, with the least friction, then that’s the right module. If somebody likes their modules to be super compact, to the expense of fine control, than that’s also totally fine.
The ethical aspect is of course more complicated and I don’t really want to get into that now. But I’ll say that there is a whole range of varying approaches and attitudes and we need to be careful to not throw out the baby with the bathwater.
Be careful when buying a cloned module. Be aware that they don’t necessarily undergo the same amount of rigorous testing as the official modules. If you have an issue, you of course need to ask the person who made the module about it. Don’t give for granted that there will be a good customer service. Which is of course not to say that all clones are badly made modules and you’re left alone with your problem if things break, what I want to say is: there’s a lot of different people making these modules, do some research before buying.