Why it had to change
Simply put, the old site was… old.
Not only the site, but everything. Starting with the OS, which was, probably, the main reason the site had to be redone. The OS reached its end of life and had to be fully updated, forcing PHP to be updated too. Which means… yeah, it would break the old site. Nothing was going to work because the site was written using an old PHP version. I am not talking about breaking a few things. I mean, the site would load just an empty page.
The PHP framework the site was built upon was also (very) outdated, which was another reason the site was loading very slowly. There was no way to just adapt its code to work with a new PHP version.
Rebuilding the site's core code also means redesigning the database. Optional, but highly recommended. Which was what I did. It took months to redesign and migrate all the data. When you have tables with almost a billion rows, and thousands of new rows being added every hour - reading, processing and rewriting all that data can't be done in one day unless you have a friend at NASA.
It is not that I wanted to change the site. I had to.