With uncontrolled migration you cannot differentiate between those who are going to contribute and those who are not.
Equally with uncontrolled migration you cannot control the flow, and so cannot plan the infrastructure to accommodate them.
Even in the case of those that are going to contribute they need provision of all the services and infrastructure (doctors, hospitals, schools, housing, social services, etc.) from day one. That is long before whatever they are going to contribute is seen. If the country was awash with money then it might be manageable, but it isn't and it's not.
Immigration needs to be controlled to allow those in that have something to offer and what they offer is needed by the UK, arrive at a rate the country can cope with, and keep out those a danger to the country.
Controlled immigration does not mean no immigration or the deportation of anyone who looks or sounds 'a bit foreign'. It has already been stated than in the event of a Leave vote that the intention is that anyone legally domiciled here under current EU free movement law would remain.