That would defeat a major goal of isolation: security. It also creates fragility. What happens if a process is buggy and rallies up to 100% CPU? It affects all

That would defeat a major goal of isolation: security. It also creates fragility. What happens if a process is buggy and rallies up to 100% CPU? It affects all others. Although to solve these issues you could use cgroups and namespaces... Aaaand we're back to containers again.