Working out the cost of the loan would have to take into account any tax relief that was claimed when paying a mortgage, but I'm sure a formula could be arrived at that would do that, as well as factoring in inflation, average wages and other relevant figures. The fact would still remain, however that those who bought at a particular time in particular areas have made capital gains that were not a result of work.
I agree that the lower threshold for paying tax should rise, as should the amount paid at the top rate, but that still assumes that it is those earning an income who should bear the main burden of tax, when workers also contribute their time and the goods and services their work provides, and pay for the ability to do so - in commuting costs, childcare charges, work clothes etc. I would prefer to see a fairer way of collecting tax from everyone able bodied and not caring for the sick or disabled, so a choice not to earn an income does not equate to a choice not to contribute financially to the benefits of living in a civil society.
I don't think that a radical change like that could reasonably be imposed retrospectively, however, as people make choices based on the prevailing circumstances, but IHT is rather different, as by definition it takes nothing from the people paying it.