Motorways are not only for local traffic. The need (or lack thereof) for an expressway link between the planned R43/R35 intersection and the A8 south of Wroclaw will become more clear (one way or the other) after the completion of the R52, the Austrian A5, and the Polish S5. However, I think the next step after completion of the planned construction along the Poznan - Wroclaw - Klodzko - Brno - Vienna/Bratislava corridor would probably be a continuation of the Polish S8 to Klodzko, before a northward continuation of the R43.
I can't see Poland even thinking about a southwards extension of the S8 for a very long time - the Government won't fund it, the provincial government can't afford it, Wroclaw is way too much in debt to help and so on. They're struggling to secure the funding to get the S3 built from the A8 to the border - and S8 is very much behind that.
The S8 does make sense from a development point of view - places like Niemcza are dreadful and a fast link to Wroclaw would really open them up for development. But there are just far more important roads to be built.
The interesting question is the future of the Wroclaw-Vienna route. S3/R35 to Hradec Kralove is certain, but after that? Putting traffic through Prague is a dreadful idea, so the only logical outcome is a new expressway/motorway from Parbudice to Brno. But whether that will happen is another question.