On the contrary, a growing economy increases prosperity for everyone. Just look at what happened in your country in the 19th century. Were people worse off in 1900 than 1800?Like virtually all progressives Zohran Mamdani does not understand the contradiction between promoting open borders and making America more affordable for working class and middle class Americans.
More people, whether from a high birth rate, a high immigration rate, or both, deflates wages and inflates rents.
If you think there's "no more room" for new arrivals, compare the population density of your country to a place like Germany or Japan or France or the UK or basically any other advanced nation! (On the other hand, don't bother: without immigration, your country's population would be decreasing)