The Makedonian phalanx -- why such depth?

[Paralus]

… so that when 'closed up' into 'pyknosis' the line is just 4 deep, with each man on a 3 foot frontage, as Xenophon famously tells us. The front 3 ranks of Hoplites could use their spears, with a rear rank of 'file closers', again not wasting the 'fighting strength' of the formation.....

Xenophon specifically says the men were drawn up in "Battle Formation" 4 deep, and that it was in this formation that they carried out their mock charge, which was sufficiently real as to terrify the on-lookers into running away!!

Again, the Greek term you refer to as “battle formation” or “closed up” is en machen. This is precisely the Greek descriptor utilised by Arrian for Alexander’s dispositions at Issos as advances the 30 odd stades in battle order / formation (“en machen”)

Yet you have totally dismissed this earlier in the thread claiming it means “open order”.