Never said it was positive, its only positive if you get migrants with the skills the country needs which I think is what the studies show. The idea that we welcome everyone with open arms because a number of them might be a doctor or scientist is misguided.
In any normal market, all of the roles we cant fill in things like the care sector should drive up salaries until they become attractive to people, but instead we have gone down the route of suppressing wage growth in lower skilled roles in favour of importing cheap labour. That cheap labour, particularly from the poorer parts of the developing world brings their dependants who then need supporting and are entitled to benefits along with the cost of housing, schooling etc. as the main wage earner is employed.
You then have someone who from UK PLC point of view financially providing a net negative contribution, whilst also having to pay to support the UK resident who decides that the job doesn't pay enough to warrant coming off benefits and into work. All of which causes more pressure on housing, health care, education etc.