“Jehovah's Witnesses and many other religions believe Jesus will return or has returned (invisibly).”
Many Christian sects believe in the return of Jesus, but invisibly? Only the JWs believe Jesus will return invisibly. That way it’s easier to prove. It’s like transubstantiation. The emblems change into the literal body and blood of Jesus. Catholicism uses it in place of the gifts of the Spirit. When healings, visions, and speaking in tongues ceased, the church needed to replace them with miracles of their own. So what better way than to come up with an invisible miracle? Every week the priest performs a miracle and he doesn’t have to prove a thing. It’s like the fastest gun in the West routine. Wanna see it again?
“If Jehovah was actually planning an Armageddon, then why is he waiting so long? I think, that it's an obvious fact that the world's population is constantly increasing. Why would you wait so long, which means just having to kill more unbelievers? It's been over 2000 years now. What is he waiting for?”
You proceed from the premise that Armageddon is as the JWs describe it. As recorded in the Bible, it’s not the unbelievers who are destroyed; it’s the wicked. There are many in the world who are good, decent, honorable people, and these will be spared. How do we know this? The Lord says that of the army of Gog (Ezekiel 39) left after the battle (Armageddon), a sixth would be left. In other words, Gog (the “beast”) will be destroyed, and 5/6th of his army. But the sixth part will be spared. Why? Are we to assume that they’re Jehovah’s Witnesses? Or that they’re “believers”? No. When Jesus returns, a large part of the world will still have not been taught about Christ. But many will be honorable people. When Jesus returns, it will be his glory that destroys the wicked — for no man can see God and remain in the flesh unless he is quickened, as Moses was quickened when he saw God face to face. This same glory will only destroy the wicked on the earth.
As to why the Lord is waiting so long, he really isn’t. It only seems long to you. According to the scriptures, the Lord created Earth in six days. This isn’t 24-hour days but six eras. Similarly, the earth’s existence is divided into seven “days” (with each day being a thousand years). Since the Lord finished his work and rested on the seventh day, the earth’s temporal existence finishes on the seventh thousand-year period which is the Millennium. We say it will be a thousand-year period of peace, but it’s really not an entire millennium. Jesus returns in the beginning part of the Millennium and then establishes peace. So since Adam, we are still in the end part of the sixth thousand years. At some early part of the Millennium, a war is launched against Jerusalem and Jesus appears in the heavens and delivers the Jews from their enemy. It is then that Gog is defeated and the Millennial reign is instigated. At the end of the Millennium, there is one more Satanic rebellion. It is quickly put down and Satan and his angels are cast out for eternity. At that time, death and hell are destroyed, the judgment completed and Jesus delivers the glorified earth to his Father. Thus, it was determined that mankind would continue through seven millennia.