So the comparison is a low-skilled migrant against an average British-born worker? Do you see the flaw in that? Even if they both end up not being net contributors, you are not comparing like with like, but you are also making assumptions about migrants having low skills. How does it take account of the fact that we are deliberately courting "low-skilled migrants" to fill our care homes with workers. Or what about the highly skilled but relatively low paid medical staff we deliberately recruit because we need them? They are also migrants. The vast majority of migrants do not arrive on small boats. They don't even claim refugee status.
You also need to take into account that lower paid workers of any ethnicity tend to die younger, so many won't make it to 80. Longer lifespans are directly related to income, education and housing.
Although the government has never kept records - perhaps they should start - a range of research shows that a significant portion of refugees possess high levels of education and skills, such as degree-level qualifications and professional experience, similar to the general UK population. I personally know three medically qualified doctors who can't work as doctors because somehow we can't figure out a way to recognise or convert their training. We have utterly no need of doctors though, so that is fine. A friend works as a taxi driver. He's a physiotherapist with nearly 15 years experience. Can't get a job because apparently we don't need physiotherapists. Two women who are nurses, can't get a job because again, their qualifications somehow aren't quite right, yet we are importing nurses because we don't have enough, all of whom also qualified elsewhere. There are a couple of people who simply can't find work and have been trying really hard. Apparently putting your life on the line to work with British forces isn't a qualification in anything useful - both of them have degrees and qualified as teachers. No teacher shortages in the UK though.
Quite apart from the fact that migrants and refugees are not the same - some refugees may be migrants but all migrants are not refugees - there are loads of assumptions being made that simply don't hold up to scrutiny. In 2024 only 16% of migrants arriving in the UK claimed asylum.