Applying updates without degrading service has always been somehwat difficult to do; but it can be done in ways similar to "graceful reloading" that is relative

Applying updates without degrading service has always been somehwat difficult to do; but it can be done in ways similar to "graceful reloading" that is relatively widely used in web servers. However, it requires custom integration work, which means you need a capable sysadmin and perhaps a developer or two to implement it, so usually you just don't. Clean restarts also avoids bugs that you might introduce there.