I've been discussing this with my AME. Apparently an ECG only shows you haven't or have had a heart attack, not that you might have one in the future.
There is no non-invasive procedure to predict this. It's not just flying instructors who fly with non-aviating people (as this instructor just had prior to this flight). There are plenty of Public Transport operations in the UK flown by a single pilot - in Islanders for one example.
Should we save time, effort and money and do away with ECG? Possibly.
A former colleague of mine was banned by the CAA from flying with students who hadn't already gone solo due to an abnormality with an ECG. This was decades ago and he's lived to a ripe old age. Actually, he took up instrument and twin instruction and found this far more rewarding than ab-initio instructing, so no harm done.
So the take away is: any of us could croak at any time. Nothing can be done about it.
TOO