Exactly. There isnt enough food for everyone to have an awsome diet. People dont like to starve to death, and they want a fairer distribution of the now very limited resources. Noone deserves food more than anyone else and the sustainability of the ark is quickly failing...so they want a plan B. They want a sustainable option.
This would involve finding other resources, which is a great risk versus an absolute risk. However, a subgoal to be accomplished would be to find out how much food there is, if the raw data was available then there would be no argument. Again, no one would expect those in power to have a ridiculous abundence of food which they're withholding to be funny, but one could expect them to be eating much better then the guests. This isnt an absolute, but they feel the truth needs to be examined because people around them are dying.
There is enough foor for just about 5.000 people. If you split it to equal rations for 50.000 people,
everyone starves to death sooner than later.
Fishing the waters around the ARK empty is a not very thought through option, because it will as well only be a temporary solution. Plus it will probably be already part of the 5.000-people supply.
The subgoal you're talking about is -basicly- already known. Though there is no actual number (and depending on seasons, there will probably be different ressources available), it is clearly not enough. Hence why there is an upcoming Civil War.
Also the original 5.000 people (which, I guess, are mostly members and or supporters of the Security-Faction) will have slightly more ressources, but they still have only enough to survive for a longer time than the Resistance-People.
If 50.000 people stay on the ARK, it boils down to a breakdown of the system.
and on the note of the population control thing, its marvelous but it obviously didnt happen so one must ask if the founders wouldve even done it themselves.
Probably because over 20 years there will not much generation-change be going on.