Hey!
There is actually a thread you might want to check out on the topic you address: What are your tips for running "Juju in production"?
From my own experience, juju is very much ready for enterprise grade deployments. But as with any software service of significance, it comes with effort and knowledge. Juju will require hard work from your crew, but will also pay back once you have made the needed investment.
The scale of your operations (10k apps, 500+ dbs and a few machines) is not a limiting factor here. In fact, I think that perhaps its describing a situation where Juju would allow you to comprehencibly manage that workload.
There are also professional help you can find in this community if you plan to move ahead with this.
On the note of LXC and vmware, juju is able to consume all of those underlying technologies as “clouds” on to which you then deploy software. LXC being the container class infrastructure, vmware fully virtualized and other clouds, such as MAAS would be a physical cloud. To juju, they are all viable targets.
Hope it helps and feel free to reach out if you need to.