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The whole point of the supernodes is to be the backbone of the system. You wouldn't want people who were chopping and changing all the time as they'd add nothing but instability.
And what is more important those coins are laying onlie outside exchages while most POS coins live on exchange.
Those POS coins stack hard on bttrex or polo. When such exchange is hacked it can cause big problem for any POS coin community.
You remember what drama was with NXT,VRC when hackers got those coins high chunk of it.
In NEM such supernodes are great we have 300x3 ~900M coins guarding network this is probably about 15% of all coins available is constantly online.