Most of the claims you've made seem to be coming from the existing paradigm of how people write code for the cloud. If you're determined to stay in that mindset, then unikernels will probably not make much sense to you. The whole point is to reevaluate our assumptions about developing for cloud-native environments, which leads to different approaches (many cloud-services are already single-purpose). No-one is claiming that unikernels are some kind of panacea but they are a useful addition to the toolbox.
Also, if you really want to compare security and vulnerabilities, then you should consider things like TCB and look at CVEs for both Xen and Linux. The way you describe it is almost disingenuous. The following talks have useful discussion points.
https://fosdem.org/2015/schedule/event/zombieapocalypse/
https://media.ccc.de/browse/congress/2014/31c3_-_6443_-_en_-...