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The current powerdown time is also discourage new investors to stake.

Most importantly it discourages most of our HIVE community members to power-up.

I strongly believe that exchanges have too much money that 13 weeks or 5 weeks makes no difference for them.

Somewhere between 5 - 10 weeks staking is what I wanted to see

4 or less is good. you twisted my arm so ok, I'll settle for 5 :)

And if the question is: why should an exchange be able to Power Up?

How do you stop them?

You cold stop it from one account, but an exchange could just set up another.

I dont think there really is a way to prevent then from doing that. It is part of all hive wallets.

Maybe with an agreement and coding. Is the blockchain consensus that should make the "rules" not the Exchange. Or should we be on the hands of a exchange to have an exchange listing hive?
I know this can sound a bit arrogant but is not my intention, what I mean is we (all hivers and the blockchain it self) must count more than an exchange and never more be in risc with a "stake attack" like it happened on Steem. Now it take 1 mouth to that "stake attack" but if for some reason happens we have to take some action, so maybe a pre agreement may be accepted by exchanges, don't know..
Anyway.. I'm comfortable with actual power down, just not sure if potential investors are.

You could give recognized exchange accounts other privileges in return for losing voting rights. E.g. listing them on the front page or in a wallet menu, or paying them interest.