What you are saying about Christians who are not going to church are not saved is utter rubbish. It's all one-sided. I Kings 18:21 tells us not to limp between two opinions. Amos 3:3 says how can two walk together accept they be agreed. II Corinthians 6:14-17 says come out from unbelievers -- those are Christians not non-Christians. II Timothy 3:5 says having a form of godliness denying the power from such turn away, don't turn to them. Revelation 3:15-16 is about the church being neither cold or hot but lukewarm spewing them out of your mouth. Revelation 2:4 says having something against you because the church has lost its first love. You don't turn to a church like this, you turn away. So are these Christians off the hook? Yes, they are when it concerns the state of the church today.
Your argument is that because you imagine some vague, unspecified fault among what I assume are denominational churches you are given the privilege of ignoring the commands to assemble as a church. The reasons a Christian is required to assemble and worship with other Christians are spelled out in "Do you think it's a sin not to go to church?" and "Does a person have to attend church?"
The answer to Elijah's question, "How long will you falter between two opinions?" (I Kings 18:21) was not to stop worshiping God. He challenged the people to leave idolatry and return to true worship. "If the LORD is God, follow Him!"
It is God who asks, "Can two walk together, unless they are agreed?" (Amos 3:3). He asked this right after warning the Israelites, "You only have I known of all the families of the earth; Therefore I will punish you for all your iniquities" (Amos 3:2). The question is a warning that the Israelites cannot claim a relationship with God while going against His will. Thus, they cannot expect leniency from God just because God had picked them from all the nations of the earth to be His people.
Paul stated, "Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?" (II Corinthians 6:14) as a warning that Christians cannot blend worldliness with Christianity.
Paul also warned, "But know this, that in the last days perilous times will come: For men will be lovers of themselves, lovers of money, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, unloving, unforgiving, slanderers, without self-control, brutal, despisers of good, traitors, headstrong, haughty, lovers of pleasure rather than lovers of God, having a form of godliness but denying its power. And from such people turn away!" (II Timothy 3:1-5). Again, the warning is to stay away from evil and worldly people.
The church in Laodicia had compromised its position. "I know your works, that you are neither cold nor hot. I could wish you were cold or hot" (Revelation 3:15). It was Jesus who was warning them to straighten up or be rejected (Revelation 3:16). He is not telling the brethren in Laodicia to abandon their assembles, but to change. "As many as I love, I rebuke and chasten. Therefore be zealous and repent. Behold, I stand at the door and knock. If anyone hears My voice and opens the door, I will come in to him and dine with him, and he with Me. To him who overcomes I will grant to sit with Me on My throne, as I also overcame and sat down with My Father on His throne" (Revelation 3:19-21).
Yes, the church in Ephesus had left their first love (Revelation 2:4). But again the solution was not a mass exodus from the church, but a change in behavior as a church. "Remember therefore from where you have fallen; repent and do the first works, or else I will come to you quickly and remove your lampstand from its place--unless you repent" (Revelation 2:5).
In every case you cited, the problem was sin and the cure was to abandoned sin. Never did God suggest that people could do less than what He called them to do simply because others were not living up to His standard. Yet this is the very thing you claim.
If there are sins in the church, then it needs to be removed. If a church won't leave the world behind, then it is time to fine one that lives by the Gospel. If none are faithful, then start one with other, like-minded people. But never is there a call to abandon the assembling of Christians.
"Let us hold fast the confession of our hope without wavering, for He who promised is faithful. And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching" (Hebrews 10:23-25).