I am actually a pro immigration person but I do think that there are cultural issues with relying on immigrants to replace all the people not being born and assuming they will keep society running and pay pensions they way you like it. The current stats look like we need 1 immigrant for each person born here.
immigrants are less likely to be loyal to the existing culture or the older generation. It's not thier Dad or Gran affected by thier voting choices.
For instance, if your culture is one that is big on multi generational living, you might vote for things that support this over nursing homes or care at home. Which is fine but our current culture isn't like that and it could mean all those childless future old people having fewer options as its just not on the radar of the now biggest group of voters. I dont mean they would be cruel, but just a different mindset.
Or with the state pension we have this social contract. I saw my parents support thier parents, now it's my turn, then my kids. I feel if you turn up aged 30, you hear 'there won't be a state pension' when you retire, or you aren't even intending to retire here at all as you have links to other places, what's the incentive to keep a decent pension going when you vote.
Our current culture is quite based on mothers working with mat leave to support that. This could easily be changed if the majority felt mothers should be home.
So immigration is fine but you can't really expect a population of immigrants to keep the existing culture or values and it really only takes small bits of the electorate to swing elections.