Hmmmm.
Lets see. Without mass immigration, there would have been no Rotherham, no Oxford, no Rochdale, no Telford.
No 2005 train bombings, no 2017 bombing at Manchester Arena.
Wages would have been higher, uber drivers, just eat drivers and online delivery drivers would likely be youngsters starting out on min wage - modern day pub working or pizza delivery. Not illegals working off their immigration debt to criminal gangs as debt slaves.
Social cohesion would undoubtedly be better, Brexit almost certainly would not have happened, Tommy Robinson wouldn't be able to get 4 people to Tescos, let alone the 500k- 1m we saw the other week.
People wouldn't be arguing over immigration. Politics could have spent the past 15 years on something constructive.
You are right. The capitalist system requires population growth. And governments have chosen immigration over births. But That's a choice you support, so it wouldn't change under socialism, much the same as now.
But let's not pretend it wouldn't have made a difference if they had gone for long term population growth over immediate.