> Nowadays they have been written 50 years ago.

Small nitpick: they have been maintained since 50 years ago. Very few programs written 50 years ago have spent the last 10 years without some sort of patch.

Of course. But as noted above that maintenance has been very cautious, because no one wants to unduly disturb the running system.

My grandfather made good money in the late 1990s patching up ancient systems for the y2k bug, before his retirement.