The main developer was an old mechanical engineer who learned coding by himself. And to be simple, his code was simple. There was a GUI with a 7x8 table of numbers. How to do this? 54 text fields, all with their own name with copy pasted initialization. Most variables were global, no class hierarchy, no fancy algorithms, juste nested loops, etc...
Because it was so straightforward, it was surprisingly easy to understand despite its complete disregard of any methodology. If it has to do A, you will see A, no surprise action at a distance, and if it has to do A ten times, you will see AAAAAAAAAA. But maintaining it is as bad as it may seem, and if you have to change A to B, you will need to change all ten of them, being careful that they may be slightly different, and forget about parallelism, security, undo, etc...
So yes, simple code is simple, but without at least some level of abstraction it will soon end up being terrible. What is too much or too little abstraction? Just hire a few world class developers and give them as much time as they need, because it may be the most difficult question a developer has to answer. In fact, one could argue that the mythical "10x developer" is one who answers right.