"One sensible interpretation for IFR for a country would be, if everyone in the country were infected, what percentage would die."
Yes, that is what an accurate IFR will reflect if it has been age and population adjusted.
Ignoring the over 70's is a very odd/ selective way of looking at it given that the biggest risk factor is age and the IFR increases dramatically with age. It's like saying yesterday was a lovely sunny day if you just ignore the fact that it pissed down with rain all afternoon!
An overall IFR doesn't tell you much anyway, for people under 20 years old the IFR is practically zero, but for a 71 year old it's much higher than the overall figure.
Rather than focus on overall IFR, it's more useful to look at the age-mortality curve. Variation in overall IFR is largely due to population age distribution and who gets infected. So for example countries with a population of lower average age than others should have a lower overall IFR.
The IFR that is relevant to the UK and similar countries, will be completely different to one for countries whose population have a much younger average age.
As example for the UK, the large serological study done recently in the UK gave an IFR figure of 0.9% excluding care homes. (The figures are adjusted to allow for those who did not produce antibodies or whose antibodies have faded.)
mobile.twitter.com/DevanSinha/status/1293830021199679488
For an age based breakdown of likely IFR, see the figures here for an example:
mobile.twitter.com/GidMK/status/1300644364117356545
Of course it's not just an outcome of you will either die or you will be fine, a minority will become seriously ill but recover and some may experience longer term illness, even from relatively mild or moderate cases. How many and how long term we don't yet know but it's a reason to remain cautious.
As others have said it's not just about the deaths but the pressure placed on healthcare by people who get ill enough to go to hospital. Average age of ICU patients is around 60, in other words half the patients are over 60 but half the patients are under 60.