I seem to remember it's "wherefore" rather than "therefore". But the real philosophical question is: given the premises (the burden of the carol), wherefore comes the conclusion? Nothing that happens in the song leads one to suppose that "Ye who now will bless the poor, shall yourselves find blessing".

I prefer the other version:

Good King Stephenlas looked out
On the feast of Wencelas
A snowball hit him on the snout
And nearly knocked him senseless.
Brightly shone the stars he saw
And the pain was cru-el
Till a doctor came in sight
On a cross-eyed mu-el.

Jon Bartlett