The challenge with IHT is that the best way to avoid it is to have WAY more than enough and hence be able to give away large sums to family members while still living. IHT will only get applied to whatever the person still owns when they die.
Whereas people with somewhat more than enough are more likely to keep it all, and then pay.
Example. Person A has £2 million at age 50, person B has £100 million. They each die unexpectedly at 60. Person A may think they need to keep all of it since that yields a sustainable annual cash flow of about £50-60k, which though not small, definitely does not make them rich. So they keep it all, and the estate pays full IHT.
If Person B spends double what person A spends, they need to keep about £4 million, but they can give away £96 million. Lets say they give away £90 million, and die 10 years later. Their estate is larger at £10 million so there is more IHT due, but £90 million went to the next generation with no IHT.
Realistically the only ways to tax that are (a) annual wealth tax or (b) tax large gifts as if they were inheritances, at the moment of the giving, instead of letting them be treated as "potential exempt transfers" which is how Person B avoided any IHT on the £90 million of gifts at age 50.
Annual wealth tax can work really well in countries with a very stable tax code (think: Switzerland) but most countries have not been able to implement a wealth tax and have it stick. Taxing gifts can be done but is not popular and can cause even more exodus of the wealthiest people, since if they leave the UK and then make gifts several years later, they would avoid the IHT.
One proposal out there (I believe from Labour) is to make IHT applicable to a UK resident who dies within 10 years of moving away. So the estate of a person who leaves at 50 and dies at 59 would be subject to full whack IHT by the UK. It will, however, be interesting to see if and whether that can be enforced.