page router isnt old, its still used today. Its technically more battle tested than the app dir.

If you are more confident in using something thats been used for over a year, go for pages dir. There are far more tutorials in pages dir than app dirs.

However, if you want to take advantage of more of react's latest feature, such as suspense, streaming, partial rendering, etc those are only available in the app dir.

There aren’t many drawbacks to using either. Most people don’t take full advantage of app router tbh, like the other commenter said. Unless you need the more advanced features, app router and page router are just stylistic preferences for code structure.