Ultimately, all isolation has its limits. Traditional VMs suffer from hypervisor exploits. Dedicated machines suffer from network-level exploits (network card firmware bugs, ARP floods, malicious BGP "misconfigurations"), etc. You can spend an infinite amount of money while still not bringing the risk to zero, so you have to deploy your resources wisely.
Engineering is about balancing cost and benefit. It's not worth paying a team of CPU engineers to develop a new CPU for you because you're worried about Apache interfering with MySQL; the benefit is near-zero and the cost is astronomical. Similarly, it doesn't make sense to run the two applications in two separate Kubernetes clusters. It's going to cost you thousands of dollars a month in wasted CPUs sitting around, control plane costs, and management, while only protecting you against the very rare case of someone compromising Apache because they found a bug in MySQL that lets them escape the sandbox.
Meanwhile, people are sitting around writing IP whitelists for separate virtual machines because they haven't bothered to read the documentation for Istio or Linkerd which they get for free and actually adds security, observability, and protection against misconfiguration.
Everyone on Hacker News is that 1% with an uncommon workload and an unlimited budget, but 99% of people are going to have a more enjoyable experience by just sharing a pool of machines and enforcing policy at the Kubernetes level.
I agree with most things you’ve said around gVisor providing sufficient security, but it's not just about security, noisy neighbors are a big issue in large clusters.