Active users is the standard metric used to check how much a service is used (at least as far as i know. its what i see when i look at stuff published for investors).
hexbar is on the sixth place in term of number of active users with 1.8K , lemmy.world is 18K (enable the "active users" column and sort by it to see the full list)
The only thing certain is death and taxes . but unlike for example omegle and it's successors you have to register so if it will be detected that it is a LLM it could be banned (eventually people will find out because LLM are not that good). Also i assume someone will have a incentive to do that so eventually it will try to get someone money or something like that and will probably get banned.
You can also say the same thing for every platform you communicate with people like lemmy . you might even wonder if a for profit company might have better resources to detect bots.