Enterprises don't change hypervisors like they change their socks...
Background: I am a Solutions Architect for Red Hat. I work with many customers from small to large enterprises. I have worked with many organizations that have expressed a desire to move away from VMWare.
First, a point I would like to highlight. There is a difference between Hypervisors (eg: ESXi, KVM, Hyper-V, Xen) and "Virtualization Platforms" (eg: RHV/oVirt, Proxmox, vSphere, OpenStack). Hypervisor is an underlying technology that implements virtualization, whereas the platform manages and integrates that hypervisor with automation, monitoring, scaling, networking, resource management, etc.) Almost all businesses need a Virtualization Platform, not just a Hypervisor.
Of the organizations I have worked with, most use VMWare, some use Red Hat Virtualization (the Product that for which oVirt is the upstream development project, and which uses QEMU/KVM under the hood), and others use OpenStack, HyperV, Proxmox, Nutanix and more. Some organizations use multiple solutions.
Having worked with all of these organizations for many years, here is my observation: With the exception of very small deployments, it is exceptionally difficult to create a compelling business case to change hypervisor platforms. It is like changing the engines of an airplane in-flight. The longer an organization uses a technology, the more its operational process becomes integrated and dependent on that technology. Many here have already pointed out integration with backups. But there is also integration with networking, ITSM systems, automation, and applications. There is training, operational experience, and knowledge in how those systems work, and their ties into other parts of the business.
The magic isn't in the Hypervisor (though, frankly, the VMWare Hypervisor is very good), it's in the systems that manage and integrate that hypervisor, such as vSphere, NSX, vRA/vRO, etc.
So, alas, most of these 'alternatives' aren't really alternatives for anyone other than a hobbyist or small business that doesn't have to manage VMs at scale. I would say Proxmox is the most likely option for anyone small. There are some large deployments of Red Hat Virtualization (oVirt), and it's a pretty solid enterprise virtualization platform. As others are noted, Red Hat's focus is shifting to OpenShift Virtualization (based on the upstream KubeVirt project), so the oVirt community would need an infusion of non-Red Hat contributors to get wind back in its sails. KubeVirt-based solutions aren't yet ready for many of the use-cases that are currently filled by VMWare. In fact, some of those use-cases may never be a good fit for container-based virtualization, as the approaches are not the same, even though the underlying hypervisor is.
With all that said, what do I recommend to organizations that want my opinion? Don't think about your virtualization platform in isolation. Think about your business, your apps, and what value you get for what you pay. If your virtualization platform costs a lot of money, but also delivers a lot of benefit, and the cost of switching it out doesn't give you a business advantage, why do it? On the other hand, if your business is being hampered by mountains of technological debt, you have poor reliability, or other challenges, maybe it makes sense to plan a future that takes advantage of newer technology, uses more products that use an open-source development model, etc. Don't run away from something you think is bad, plot a course to something better.
But whatever you do, don't just 'lift and shift' your VM's to the cloud because of a belief that it will 'save you money'. That is almost never true in the long run.
Biting the hand that feeds IT
