"between 2007 and December 2014"
That's seven years.
Over seven years the company has largely enough time to train somebody in-house, get a knowledge transfer or acquire some expertise and bring it in-house.
A contractor that lasts seven years is basically part of the woodwork. That's exaggerated.
HMRC's approach to the problem is exaggerated as well, but seven years is a hell of a long time to not be part of a company you work at every day of the week.
I've been a contractor for the past 25 years. I go in for a day, for a few days, a week, a few weeks. Rarely, for a few months. In all my career I've had two contracts that lasted around 18 months each. Those were very long contracts and I was glad to see the end of them.
Seven years ? I think I'd go crazy. Hire me already and let's stop pretending.
That being said, it certainly smarts being hit with £75K of back taxes. My sympathy to the guy who just tried to make a living.