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It isn't an altcoin if it uses the same blockchain, same client without a restriction on the blocksize
At the moment BU contribute between 5 and 10% of the global btc hash rate..... You can see the charts if you Google for them
This is not about BU. Either SegWit is activated or not. If its activated wallets must have code to handle the new SegWit transactions.
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It can't start working automatically without doing the coding for it otherwise Electrum tweet wouldn't make much sense.
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Judging from this tweet[1] MultiBit HD is either already SegWit ready or at least they are working on it. As you can see from the date its been a while since KeepKey bought the wallet.
[1]
https://twitter.com/delrayman/status/735531413005598721