To begin with, some preparatory information: On this post I have a number of links to informational posts regarding when Jesus began ruling. This information is important because in the account in Revelation 12, the "child" that the Dragon is trying to devour is 'caught away to God's throne' before the Dragon (i.e. the Devil) is warred against and cast down to earth.
From that point, if one accepts the idea that Jesus was enthroned as king when he returned to heaven in the 1st century then:
Satan’s ouster from heaven is depicted in Revelation 12 directly after and as a result of, 'the woman's child being caught away to God and to His throne (an oblique reference to the start of the 'child's' rule). This would tie Satan's ouster with the enthronement of Jesus, shortly after he returned to heaven. (Revelation 12:5-9, 10)
One might counter-argue that after Satan is cast out of heaven, he is "having great anger, knowing that he has a short period of time.” (Rev 12:12 NWT) So one might argue that if he was cast down to earth in the 1st century, how could it be said that he "has a short period of time" left? In response to that, one might point to Rev 1:3 and the fact that, "the appointed time is near," even though we are now 1900+ years from when that was said. (Compare also Rev 2:16; 3:11; 22:7, 10, 12, 20) Thus, one has to take "near," "short" and "quickly" in perspective. (See also this post for additional discussion of Satan's "short period of time.")
This view, that Satan was ousted shortly after Jesus returned to heaven, would correspond nicely with the fact that Jesus, while on earth, said that this would be an event then soon to take place, and he linked it with his death:
Regarding “now” in John 12 (Greek nun, Strong’s #3568), AMG’s Greek WordStudy Dictionary understands it in this verse (as also with Acts 13:11; Phil 1:20; John 16:5; Acts 26:17) as meaning, “In reference to future time just at hand. . . As implying what is immediately to take place.” (p. 1019) Similarly, BDAG (Bauer’s 3rd Edition Lexicon) defines it as “a temporal marker with focus on the moment as such, now.” And with regard to John 12:31 (and the other passages referenced in the AMG reference) as “of time shortly before or shortly after the immediate pres[ent].”(John 12:31, 32 NWT) . . . Now there is a judging of this world; now the ruler of this world will be cast out. 32 And yet I, if I am lifted up from the earth, will draw men of all sorts to me.
Notice also how the "judging of this world" and its 'ruler being cast out' is linked by Jesus to his being "lifted up," a reference to his being killed. This ties together nicely with how the sequence of events are described in Rev 12:5-10 (when one accepts Rev 12:5 as a reference to Jesus' 1st advent and subsequent ascension).
Consider also John 16:11 -
Verse 11 is the verse of interest. But I included from verse 7 for context. The verb “has been judged” (in verse 11) is “perfect,” a completed action. Alternate renderings of the verse at BibleHub bring this out also. (See here.) Although this verse is indirect evidence, it would make plain why Jesus would say in John 12:31, “now the ruler of this world will be cast out.” There was no reason to let Satan remain in heaven for another 1900 years. He had already been judged.(John 16:7-11 NWT) . . .Nevertheless, I am telling YOU the truth, It is for YOUR benefit I am going away. For if I do not go away, the helper will by no means come to YOU; but if I do go my way, I will send him to YOU. 8 And when that one arrives he will give the world convincing evidence concerning sin and concerning righteousness and concerning judgment: 9 in the first place, concerning sin, because they are not exercising faith in me; 10 then concerning righteousness, because I am going to the Father and YOU will behold me no longer; 11 then concerning judgment, because the ruler of this world has been judged.
Also note that Satan’s ouster from heaven is connected to his having been an “accuser of our brothers.” (Revelation 12:10) Paul, in his writings, indicated that no one then was in a position to “accuse” Christians:
These verses make a lot more sense if Satan has already been cast out of heaven in the 1st century, as described in Revelation 12:7-12. In effect, Jesus' sacrifice provided the legal basis for throwing Satan out of court.(Romans 8:31-34 NWT) . . .What, then, shall we say to these things? If God is for us, who will be against us? 32 He who did not even spare his own Son but delivered him up for us all, why will he not also with him kindly give us all other things? 33 Who will file accusation against God’s chosen ones? God is the One who declares [them] righteous. 34 Who is he that will condemn? Christ Jesus is the one who died, yes, rather the one who was raised up from the dead, who is on the right hand of God, who also pleads for us. . .
(Colossians 1:21, 22 NWT) . . .Indeed, YOU who were once alienated and enemies because YOUR minds were on the works that were wicked, 22 he now has again reconciled by means of that one’s fleshly body through [his] death, in order to present YOU holy and unblemished and open to no accusation before him . . .
See further discussion of the above points in this post.
And finally, from an historical perspective. Satan's ouster from heaven is linked to 'woe for the earth and sea.' (Revelation 12:12)
In the centuries that followed Jesus' return to heaven, the greatest empire up to that time broke up and Europe entered the Dark Ages which lasted until somewhere between the 8th and 11th centuries, according to who is doing the reckoning. Human population is thought to have declined during that period. Education basically stopped for all but a few. Wars, famines, and plagues (the 2nd, 3rd, and 4th horseman of the apocalypse, which follow the crowning of the 1st horse rider of Revelation 6) were rampant on a scale that make the 20th century seem downright tame by comparison. (As some here have pointed out.) The first video sums things up quite nicely. But the description of that video underneath it at the YouTube page provides a further good synopsis of the Dark Ages. The second video below also describes how Europe fell into the dark ages and some of its effects that match the "woe for the earth and the sea" of Rev 12:12.
Compare also the after effects described for the 'casting down of the burning mountain' in Rev 8:8-9 in this post.
On comparing life expectancies and the idea of Satan being cast down in 1914 (as per WT doctrine), and subsequent "woe" for the earth, compare that with this Wikipedia page on life expectancy. It shows a dramatic rise in average length of life in the 20th (& 21st) century in comparison with most of human history in this era. Not what you would expect if "woe for the earth" began in 1914.
On what may account for this rise in life expectancy in modern times, see this post and look for the section discussing Trumpet # 4.