Will development mode go away? Also can I use it for my site in production?

The development mode will stay free forever and you may use development mode on a production site.

Development mode does not have access to modify Namespace settings which control the behavior and appearance of the comment section. If you need control, sign up for a paid plan.

Summary:

There is no pressure, you may use development mode for free as long as you like with the default behavior and appearance of the comment section.