Dangers of moving from outside to inside IR35
"There is a fear that such a move could land them with a retrospective tax bill as HMRC may view that as an admission that they ought to have been paying higher taxes previously."
That's the nub of the problem. In the absence of clear guidance from HMRC, no contractor would want to risk staying with the same client and moving from outside to inside IR35. Consequently, there will be a huge game of musical chairs over the next few months as most contractors try to find new clients. It will be massively disruptive to business.