The Pushing Match

[Geticus]

I find that unlikely, the Thebans were heavily outnumbered in infantry (~4000), it would have been madness to adopt such "cannon fodder" tactics when facing an enemy with superior numbers and training.

The whole crux of the victory rested upon the fact that the Theban left was able to defeat the Spartan right before the weaker centre and right gave way against the more numerous and better trained Spartan soldiers opposing them.

This is why Epaminondas used the echelon formation in the first place, to keep the weaker parts of his army away from the Spartans for as long as possible. If the clash on the Spartan right had been a prolonged slogging match to grind the Spartans down, the Theban left and centre would have given way before a victory could have been achieved.

They needed a quick victory.

One might also consider Polyainos' anecdote about Epaminondas calling out to his men during the battle to give him "one more step" and they would thereby gain the victory.
Hen bema charisasthe moi, kai ten niken hexomen. (2.3)
Give me one more step, and we will attain victory.
To me it seems clear evidence that classical historians accepted othismos/shield pressure as potentially decisive in battle