I think you've made too many assumptions about how HBD has affected the price of Hive (or SBD has affected the price of Steem). IMO the creation of SBD has had little real impact on the price of Steem at any given point in time (and the same of Hive and HBD). Further, I don't believe that HBD poses an existential risk to Hive in it's current form, or with the planned changes.

Crosschain solutions aren't great because they rely on trust. I've seen these go very badly wrong. Not saying they aren't better than nothing, but an onchain stable coin is much better, IMO.

HBD does provide a useful value even now, as a semi-stable coin that gets used for commerce, and it could supply a lot more as a stable coin (which would make it more suitable as a trading intermediary). With the right tuning, I think we can make HBD into possibly the first trustless stablecoin that works well.