Only by remaining in the EU would the UK be capable of contributing to EU reform.
The UK no longer has the leverage over countries outside Europe that it used to; therefore it will not be able to make deals with those countries that are equally beneficial to both sides.
The UK is still one of the leading EU nations, judging by the size of the population, numbers of the MEPs and size of the financial contributions.
The leaders of the EU wish to lead the EU toward closer integration and have demonstrated that they does not want reform in the way that UK voters do.
Even with the threat of the UK leaving the EU, David Cameron was unable to win a single significant concession from Brussels when he attempt to renegotiate the UK's relationship with the EU in 2016.