The "Downfall" is actually
a countdown from 10 to 0.
The important thing to see is that the lower number of each pair is written twice, because of the pattern known from nursery rhymes and literature:
10 of something were doing something, one did something and then there were 9...
9 of something were doing something, one did something and then there were 8...
and so on until none of the objects is left.
For example Ten_Little_Indians - I used this example to explain each line below
It may refer to And Then There Were None novel by Agatha Christie, as the phrase appears in the riddle itself, or just the general idea of this type of a song.
forgotten means 10
Ten little Injuns standin' in a line,
I will be back in September.
September is the 9th month
One toddled home and then there were nine;
If you think I am feminine,
feminine means 9
Nine little Injuns swingin' on a gate,
I will show you stop sign.
Stop sign is octagonal which means 8
One tumbled off and then there were eight.
If you are afraid of heights,
heights means 8
Eight little Injuns gayest under heav'n.
I will throw you in the sea at twilights.
The "Seven Seas" which means 7
One went to sleep and then there were seven;
If any mis event happens (heh!, so lame),
mis event means 7
Seven little Injuns cuttin' up their tricks,
you know which insect to blame.
Insects have 6 legs
One broke his neck and then there were six.
When you are sixty and low on your defenses,
sixty means 6
Six little Injuns all alive,
don't go blaming your senses.
There are 5 senses
One kicked the bucket and then there were five;
Are you deaf? I vent my anger,
deaf? I vent means 5
Five little Injuns on a cellar door,
don't you dare forget about indie's blabber.
indie = Indiana Jones, about whom 4 movies were officially released by now
One tumbled in and then there were four.
Lets cutoff our conversation,
cutoff our means 4
Four little Injuns up on a spree,
its quack anyways.
Hendrick Peter Godfried Quack had 3 names??
Donald Duck (who says "quack") has 3 nephews??
One got fuddled and then there were three;
Let the path reenacts for us,
path reenacts means 3
Three little Injuns out on a canoe,
which we built with our hands.
We have 2 hands
One tumbled overboard and then there were two
And when we are trustworthy enough,
trustworthy means 2
Two little Injuns foolin' with a gun,
there will be only one...
one = 1
One shot t'other and then there was one;
Nonetheless we think we won,
Nonetheless means 1
One little Injun livin' all alone,
"And Then There Were None"....
None = 0
He got married and then there were none.