It’s a bit of a gamble. They have lasted this long which is a good sign but the community is small. I think it’s going to be a combination of ClassicPress making enough of the right decisions combined with how many WordPress users get alienated by WP and start looking to migrate.
As far as themes and plugins go it’s kind of a matter of attrition. On the WP side I see more plugins that say they work with both blocks and classic editors, but over time more future plugins will be block only. This could push people towards ClassicPress. OTOH it also works against ClassicPress. ClassicPress has contacted many essential plugin makers and has gotten them to commit to keeping their plugins compatible with CP. I relayed some messages from CP to the Indieweb plugins team so I think those plugins will remain CP compatible. There are also some plugins that are specific to CP. There is enough plugins to do most everything in CP for now. I guess the fail safe is, should CP fail, you can always move you writing back to WordPress or to Drupal – at least it’s not in some obscure format.