It certainly isn’t impossible. Just nowhere near as simple as some people seem to think. Nor likely.
If the Queen reaches much over 100 - as her mother did - Charles would be very close to, or over 80 and William would be in his mid-40s. In that type of situation, Charles might well abdicate. But he is not likely to do it unless he is very old - and more importantly, unless William has already had a decent chance to have his own life.
Does not the Queen herself actually have the power to control succession?