As for the French and Austria situations: perhaps I mistakenly worded my post. When saying *before*, I meant to imply that the fathers in questions eventually succeeded as sovereigns themselves. Otherwise, if one is talking about fathers of sovereign sons who never reigned as monarchs, the examples are legion ("Fils de roi, Père de roi, jamais le roi").
We have seen numerous cases of thrones passing collaterally, because the preceding monarchs failed to produce direct heirs (in the past, oftentimes because of poor medicine, high death rates, and low life expectancies). When the heir who is first in line is a brother not much younger than the sovereign, you cannot assume that the younger brother will necessarily survive the older. Oftentimes in such cases, the throne passes to the eldest surviving nephew through that brother. This happened (as just one notable example) in Prussia, where King Friedrich II (otherwise known as Frederick the Great) was succeeded by his nephew, who became King Friedrich Wilhelm II.
It could have happened in Saxony, where King Albrecht was succeeded by a brother only four years his junior. As it was, that successor, King Georg, survived him by only two years. It could have happened in Belgium, too, where King Baudouin was also succeeded by a brother only four years his junior. As it is, that successor, King Albert II, is still alive and kicking -- meaning that the nephew who had been expected to succeed the uncle would still be a king-in-waiting at the age of 60.