Many will disagree, but here goes.
The main groups wanting increased population are the war mongers and (big) business.
The former need more soldiers as cannon fodder to achieve their expansionists plans, eg Putin and Xi.
While the latter need more consumers to increase profits.
The Earth has limited resources and can only hold a finite population of all life forms.
Increasing retirement age, and thus the working-age population, can help fund the care of the elderly, as would the introduction of new technology (the technologies exist, but have not been widely applied)..