To me, this would excuse a change that was actually somehow beneficial for their own chips and just happened to break clones, but it doesn't excuse a change that does nothing for their own chips and breaks clones on purpose.
Write your own driver if you want to use the clones. It is not Prolific’s job to support hardware they didn’t design, build and test.
Even more importantly it is not Prolific’s job to support competitors who are not going to respect Prolific’s IP.
For all we know, the new driver could have been a rewrite from scratch (say to support new products that run at higher speeds than the older driver could support), and the new driver never worked with this particular clone to begin with.
Keep in mind that the driver developer may not even be in possession of a clone of this type. Clones see new versions over time just like any other product, the only thing is that since they don't advertise their true version, it's impossible to practically support them.