This is all speculation, of course. But neither The Queen nor the Duke of Edinburgh for that matter have ever struck me as persons who make personal concessions until they have become absolutely necessary. In this way HM reminds me of my own late grandmother, who until just a week before her death at age 95 insisted upon living alone in her own home, driving her own car, volunteering at the local hospital, etc., and who responded to any suggestion that she should slow down with a quiet, icy stare.
The Queen is still remarkably vigorous and active, still rides out often in Windsor Park, still regularly stands for 90 minute investiture ceremonies, etc. etc. Given that, I think she might view the suggestion of abandoning the Imperial State Crown as a concession to age, just to avoid an hour or two of discomfort each year, is something she will do only when absolutely compelled.
They day may arrive when the Queen requires walking sticks (like her late mother), or can no longer manage stairs (like QV in her latter years). That's when I suspect she will begin to make such concessions.
