The engine has engineering effort in it since the late 1990's since some of the code is that old. (I say this because I remember learning from the mod SDK back

The engine has engineering effort in it since the late 1990's since some of the code is that old. (I say this because I remember learning from the mod SDK back in the day, and I've looked into what it is now). So almost 2 decades worth of engineering effort has gone into it. Granted a bunch of that effort has been in rewriting systems which needed to be updated (like rendering).