Economists believe increasing population increases growth. It does increase gdp, but it does not generally increase gdp per capita. It can create larger markets and more specialization over time, but before it does so it also increases the supply of labor, lowers its income share, and diminishes the incentive for most technological advancement, labor saving rather than material saving. Population growth did not contribute to gdp per capita growth during the Malthusian Era. Only technology did that and population is better considered the result of growth than the cause of it.
The UK has had a stable population for over 30 years and has grown gdp per capita at rates greater than the US over that period. When population lags, investment in human capital becomes more attractive and takes up some of the slack. An aging population does slow demand growth, but still increases demand relative to supply of labor. They still need product even if they no longer need jobs. Growth slows but steadies. Transfers will rise, but government less transfers should steady or even fall with productivity as expansion is no longer necessary but sustainment still is. It does tend to be deflationary, but labor will do better than capital in such an environment. Nothing to fear, but something to expect. Need I say, deflation is (almost) always and everywhere a monetary phenomena?
In a world of more or less free trade, the question of whether population growth here is good is an anachronism. The real question lurking beneath the surface is if it were, why wouldn't population be growing on its own as a result? I am sure economists can come up with some excuses, but they fall flat in the face of evidence.