To keep old reddit alive they need to also spend developer time supporting some new features and not breaking legacy code. It’s more than just server costs.

Speculation, but it’s an inevitability. It costs money to support old.reddit, which means Reddit will certainly kill it on the day that they’ll think the backlash will be small enough.

A tale as old as computers. Here’s a very good talk about it: https://youtu.be/kZRE7HIO3vk