No, not at all. Witness, in collaboration with PoW, is intended to strengthen the Gulden network and make it safer. Thereby achieving fewer confirmations and solving all kinds of other security issues. In addition, it breaks the power of miners by breaking up the transaction validation into two pieces. There is a part for the miners and a part for the witness accounts. Both have in principle conflicting interests in this transaction validation, as a result of which miners and witness accounts cannot hold the Gulden network hostage or impose their will.
While PoS is only intended as an alternative coin distribution compared to coins that work through PoW distribution.

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