A lot of the value of vmware is in the sales channels. Why use VMware rather than QEMU/KVM? it used to be that VMware came with support. But now that KVM is owned by RedHat, which in my experience, gives way better than average support? yeah.
but, yeah. Docker doesn't solve the "take this ancient rack of failing servers and consolidate them down to one server... without updating the software" use case that VMware is so often used for.
Why use VMware? Is indeed the existential question that's facing them. For now, it's because many IT shops can't wrap their heads around the alternatives or justify the switch. Lack of skills (lots of Windoes centric shops), deep love for DRS/Vmotion/HA, deep support for fibre channel setups, etc. Btw this is arguably why VMware announced Photon recently, to go after RedHat. Eat at their Linux monopoly.
That said, VMware did basically invent x86 virtualization as we know it today, and that's justified the many billions in wealth it has generated to date. Docker is (so far) a registry and a CLI wrapper around a Linux kernel feature. It can and will be more, but it's not clear what.
but, yeah. Docker doesn't solve the "take this ancient rack of failing servers and consolidate them down to one server... without updating the software" use case that VMware is so often used for.