I don't think you quite understand how difficult it is to predict the bountiful yeilds of spegetti code.
The fact is that Mord's ult is replacing other champs ults makes no sense.
Just because you write something doesn't mean you will understand nor anticipate how every bit of code will interact with other portions, and the play testers? They don't even write the code. They are hired based on their ability to play the game, not on their ability to write and understand code.
Just because they do this for their jobs does not mean they have a special ability to anticipate what things will cause problems, and probe for them(because guess what? they don't necessarily know coding!). Just to document all of Morde's bugs, the guy that has been doing that had to play hundreds, if not thousands, of hours of Morde, and he is a dedicated fan. There are 336 hours in a pbe cycle, 80 of which are business hours, when the play testers would be doing their jobs(assuming they do 40 hours a week). And 138 champs that can be potentially be affected by the changes on the pbe. If they were to be testing a champ's interactions with other champs, including things like "what happens if x champ kills y champ with z skill, while champs h,k, and t are in the game?" there just wouln't be enough time to check all the interactions! For the next champ, we are talking about 13,906,484,270,000,000,000 possible team combinations! How the hell are the playtesters supposed to test that many potential interactions? HOW?
It is far more efficient to test the obvious things that would occur to people, and let the millions of players find the rare bugs that have effectively 1/1,000,000 chance of occurring due to how specific their requirements are for them to spawn. Because spending less than a millionth of a second testing each interaction sounds like it would accomplish a whole lot of nothing.