If you make it standard procedure that everyone has to leave the country temporarily to renew their visa, the person is already outside your country when you refuse the renewal.
Instead of having a deportation challenged in court and mired in paperwork, you simply don’t let them back in. That’s what the parent comment was getting at.
Not saying this is right or wrong or it will necessarily happen in the UK, but it’s the kind of change you can make if you want to quietly reduce immigration without fuss about deportations.
And that does seem to be the direction that UK policy is going, albeit currently only with baby steps