Reddit cannot die unless their management does some insane thing that affects majority of user base. Killing 3rd party apps impacted a small minority so it was largely nothing. It is way too popular and useful to die at this point.
As for Lemmy, will be interesting to see how eventual operational cost problems will be resolved. Lemmy (Activity Pub?) is also pretty inefficient and does a lot of data duplication due to being decentralized. Centralized systems like Reddit are much more efficient.