They should have put a Kraken 2.0 trade engine alongside the first one, and moved people gradually there. It doesn't matter how confident they were with the upgrade before it happened, it's crypto, everything is new. A few lines of wrong code and you can lock millions of dollars in multi-sig wallets.
I have most of my funds there because their eur SEPA transfers worked very well. I really hope they can get back in shape after it comes back online.