IPv6 is practically built for containers, and, to Kubernetes's credit, they architected with that in mind. (Learned from BNS.) Weirdly, what I'm saying here was the original idea behind ports in the first place. There just aren't enough of them, particularly when half your space is shared with client sockets.
I want a world where v4 is pretty much just my control plane into the v6 cluster, since I'll die before IPv4. Google and far more importantly Amazon need to come up with a v6 story in their cloud offerings already. AWS has had a decade. This isn't just blind advocacy any more; the orchestration and software side is starting to build entire parts of the OSI stack because the network side of our industry is stuck without any sign of moving, no matter how dire the v4 situation.
However given Spotify's business position our priority has yet to shift from providing engineers compute capacity as fast as possible to optimising our usage of said compute capacity. It's all now somewhat of a moot point as we move away from our own hardware into Google's cloud.
It's also harder to separate two or more processes that 'grew up' together in the same container/machine/vm.
It's strange to me that this is still so common. My theory is that the "one machine one port" philosophy is still built into a lot of software (monitoring, the ELB, etc). Another is that this is the philosophy we've always known.
Take a look at Kubernetes. Everything is accessible via localhost:<some port>. that breaks most home-built and enterprise orchestration and monitoring tools spectacularly even though it's a much simpler mode (everything is a port, not ip port combo).
Density is much easier to accomplish on larger machines with more cores, which are elastic in the face of bursty residents. They are also generally cheaper per compute/memory.