Earlier quoted context omitted.
The problem with people new to the company starting a rewrite from scratch is that they often are poorly informed on why things were the way they were before. If you start big, you can have bad outcomes where the new system might be objectively worse than the old one... but you are stuck trying to get the new thing out for the next 5 years because too many people sunk too much political capital into it. As an example…
You're making a silent assumption that the original team is well informed about why the things are like they are and that they know what they are doing. I think it is not always the case. I've been to a project once where the mess in the original system was the result of the original team not knowing what they were doing and just doing permutation based programming - applying random changes until it kinda worked. The…
Particular to ad tech, the lifespan of any particular software is lower than you’d expect (unless your google/Facebook). Technology that pays out big one year will become pretty meh within 3 years. In the case above I’d argue that the new tech team didn’t really understand this dynamic and so they focused on the wrong things such as rewriting functionality that didn’t matter for the future. Or making big bets on aspects of the product which were irrelevant.
To the OP, we don’t know that the lifespan of any of these php files is greater than an individual contract. If the business can be modeled as solve a contract by putting a php file on prod - rewriting may be entirely worthless as the code can be “write once, read never”.