Very true; however the 6000psi is not present 24/7 for 2 years, and it is not present anyway without leaks. Leaks are common (which is why the fluid needs topping up periodically) and are ignored unless significant.
The vast majority of hydraulics are pressurised transiently
This is true also, but in the case of machinary it also has to contend with tempreture transients and high pressure spikes far higher than the relief's are set for. There is also higher 10000 psi systems in staionary systems.
Hence a good experienced engineer should have no problem designing for a static system.