What you're seeing is the endless see-sawing between "performance needs to be higher" and "isolation needs to be stricter". Each alternative lands on a differe

What you're seeing is the endless see-sawing between "performance needs to be higher" and "isolation needs to be stricter". Each alternative lands on a different balance at different times. Partly because of their own innate qualities, partly because of path dependency, partly because a "new" approach can sometimes reveal previously-unnoticed assumptions and partly because the ratios of CPU, RAM, disk and network performance are ever-shifting and thus favouring this or that architecture in the moment. What tends to happen is that performance gains draw the first wave to a "new" technology. Then as the bulk of the industry begins to move, isolation, robustness and backwards compatibility become relatively more important. These impose overheads that will eventually be swept away by the next "new" technology. There are still qualitative differences, though. Where you draw the firm lines in architecture affects the range of possibilities. I can place tenancy boundaries "anywhere", but in practice a VM hypervisor will be able to provide a firmer boundary than asking people not to share passwords.