I would love to have an insider's view of how (if) they'll integrate those tech stacks; the products seem close enough to make it hard to justify not combining them, but the devil will definitely be in the details.
I haven't used Five9 for about 5 years, but even then their tech seemed old. The new thing then was starting to add multi/omni-channel support ie adding chat and maybe email. My guess is that the integration will likely be adding a new video chat option for call centers (ie five9 would do the queuing and then the routing to Zoom). I'd highly doubt that Zoom would spend the time to fully combine the software.
Oh, if Five9 is far enough behind then perhaps Zoom can just tack on any needed features to their existing product; I had assumed there were enough features only on the Five9 side that actual porting/integration would be needed.