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The answer to this is obvious and is mainly history repeating itself. Sony will highly concentrate on security on PS4 AND PSN but just like microsoft they will continue to provide security updates and patches to the ps3 until most of the world finally migrates to ps4 (we're looking at 2-3 years). Once that happens, Sony will most likely only provide major security updates to the ps3 when it involves exploits to their PSN servers.
So the answer to your question: The likelihood of getting banned on the ps3 will not decrease once the ps4 comes out for at least another 2-4 years at most (Unless it involves hacks and exploits affecting the Servers). By that time your most likely going to care less what happens on the ps3 since the chances of you owning one will be slim.