You're missing the behaviour aspect. Yes, a whopping chunk is 0%, but it's the leap from 0% to 40% (on the surplus) that drives the tax-planning behaviour to try to avoid it, which for most people is incredibly easy. It's a common saying that IHT is the most avoidable tax! If it went from 0% to say 20%, there'd be less incentive for people to do tax planning to avoid it, especially as IHT tax planning often costs several thousand pounds (often with yearly costs too if a trust). If, say, your estate is £1.1m you have a whopping £40k to pay and solicitors fees of £10k seems cheap to avoid it. However if the tax was only £20k, it's hardly worth £10k in first year set up solicitors fees and then maybe £1k per year thereafter, so people would just pay the tax instead.
That's exactly what I see in my business (small accountancy practice) literally every week, whether it's the IHT changes announced by the idiot Reeves, the pension changes she'd proposing (removing IHT exemption), or all the other cliff-edges and stupidly high marginal tax rates. When people get hit by a "large" tax hit, they take steps to avoid it, whether it's working fewer hours, putting monies into a pension, setting up a trust, "giving" income generating assets to a spouse, or whatever. When they don't get the "big hits", they tend not to make big changes.
We need ALL aspects of tax to be more incremental to avoid all these "big" hikes, and by doing that, we'll strip away most of the "behavioural" often damaging tax avoidance/tax planning steps that people take.
We really shouldn't have "marginal" tax rates of a whopping 40% for IHT, or 62% for earnings breaching the £100k level, or small businesses having to register for VAT and paying VAT on ALL their takings when they go from turnover of £89k to £91k costing them thousands. We need "smooth" curves of marginal tax rates across the board so there aren't the same damaging hikes that drive behaviour which is bad for the economy/country, i.e. doctors working fewer hours to avoid 62% tax thus increasing waiting lists etc.