There is also, and if you ever coded anything you should know, the fact that projects like this can produce really messy code over time
I think the dev wanted to move to a newer engine so I'm not about to comment on their work, just on this comment.
I've read it many times and it's just not true. It becomes true only if you are a bad developer (we used to call them cowboys) that thinks maintainability is not necessary and codes in modern languages the same way it used to be in the auld days of Basic and Texas Instruments.
If you are a good developer who builds with design and technical framework in mind, it will never get really messy.
Plus it will make you much more productive.