@Nick ******
I asked immigration about this, and they told me it is just a visa requirement and therefore valid during the whole period, and that does make sense.
Nowhere is mentioned it is only a requirement at moment of approval, it is only mentioned how you might supply proof at the moment of approval for this requirement.
And immigration offices have list how you can supply proof at the moment you apply for extension.
Immigration offices cannot add additional requirements officially, they only can specify how they want you to proof you comply to the requirements.
Everybody should understand why this requirement is, and then it would not make sense it is only needed at moment of approval.
It is meant to check DTV holders have financial reserves to pay for unexpected expenses.
That unexpected expenses can occur during the whole 5 years and not even at moment of approval and also not specifically at moment of extension.
The timescale for retirement is because there the requirement is lowered during certain periods, I never really understood why, but it just is.