Let's add verses 6-8 to get a complete picture:
1 Now concerning the coming of our Lord Jesus Christ and our being gathered to Him: We ask you, brothers,
[ Here Paul sets his THEME for this passage (so they must be included in any exegesis) His coming and our gathering.]
2 not to be easily upset in mind or troubled, either by a spirit or by a message or by a letter as if from us, alleging that the Day of the Lord has come. [Apparently they received a letter telling them that the Day of the Lord had already come and they were IN IT. It must be noted that when they began to believe they were IN the Day of the lord, they were upset and troubled.]
3 Don’t let anyone deceive you in any way. For that day will not come unless the apostasy comes first and the man of lawlessness is revealed, the son of destruction. [ Here Paul is starting his argument as to why they were NOT inside the Day of the Lord. ]
4 He opposes and exalts himself above every so-called god or object of worship, so that he sits in God’s sanctuary, publicizing that he himself is God. [Here Paul tells us what the man of sin will do after he is revealed.]
5 Don’t you remember that when I was still with you I told you about this? [Paul had explained this when he was there.]
6 And you know what currently restrains him, so that he will be revealed in his time. [Here Paul tells the readers that now, after reading the first 5 verses, all readers should KNOW who or what the restrainer is, and that the man of sin will be revealed at the proper time.]
7 For the mystery of lawlessness is already at work, but the one now restraining will do so until he is out of the way, [Here Paul explains that the devil was already at work, but was being restrained until such time as he - the restraining power - is "taken out of the way."]
8 and then the lawless one will be revealed. The Lord Jesus will destroy him with the breath of His mouth and will bring him to nothing with the brightness of His coming. [Here Paul tells us that once the power - the HE - restraining the man of sin, preventing his revealing until the proper time, will be taken out of the way, THEN the man of sin will be revealed.]
Verse 1: somewhere in this passage Paul MUST show the gathering - the rapture. It is his theme.
Verse 2: Why were they so upset and troubled after learning the falsehood that they were in the Day of the Lord? If Paul had taught them that they would be raptured after all the "tribulation" then they should have been excited thinking the rapture must come soon.
On the other hand, if Paul had taught them that the rapture would come before the Day (EXACTLY what his first letter told them) and then they began to believe that the Day had come and they were in it - no wonder they were troubled. Either Paul had been mistaken, or they had missed the gathering!
Verse 3: I will sum up Paul's argument: Don't be deceived: a significant departing must come first, as the restrainer being "taken out of the way," then the man of sin will be revealed. Anyone who sees this significant departing, then sees the man of sin revealed will KNOW the Day has come and they are IN IT.
This is John's order of things in Revelation: the DAY comes at the 6th seal in chapter 6, and the man of sin is revealed somewhere before chapter 3. By that time (chapter 13) all will know THE DAY has started and they are in it. John and Paul agree.
Now lets drill down: Why did Paul write, "and now you know who is restraining..." in verse 6? The answer should be obvious: somewhere in verses 1-5 PAUL TOLD US - but perhaps did it in a cloaked manner so wrote "now you know" so the reader would go back and find it.
ANY correct theory on Paul's meaning MUST include WHO the restrainer is. The truth is, most people only guess and often guess wrong. The question is, WHERE in verses 1-5 does Paul revealed who the restrainer is? It is in verse 3, but again, with a false theory most are in error.
Why did Paul release (remove the restrainer) the man of sin in 3b? MANY English translations have "is revealed." Verse 4 proves Paul's meaning as "Is revealed" because verse 4 tells us what the man of sin will do after he is revealed. Since verses 6-8 tell us he CANNOT be revealed until the power restraining this revealing is "taken out of the way" then any serious student must ask HOW did he get revealed (IS revealed) in 3b? It should be obvious (most beginning students miss it) that somewhere in verse 3a, the power restraining the man of sin is taken out of the way. It MUST be somewhere in these words: Don’t let anyone deceive you in any way. For that day will not come unless the apostasy comes first....
The restrainer taken out of the way CANNOT be hidden in "Don’t let anyone deceive you in any way."
"For that day..." What day? Of course the Day of the Lord from verse 2. Can the restrainer removed be hidden in "for that day?" No. We are down now to the last phrase: unless the apostasy comes first.
Somehow, the restrainer taken out of the way MUST BE hidden in this phrase. If we use the KJV solution - the "falling away" then a falling away (from what we don't know) must be the restrainer removed. Frankly, I can't see how people falling away from something or NOT falling away from something can have anything to do with restraining.
Next, is a "falling away" the same thing as a gathering - Paul showing us where his theme is fulfilled? No, it cannot be.
If we just "departing" as several of the PRE-KJV translations used, what then could a "significant departing" mean? (All Greek texts have a THE before apostasia - it is not just any departing, it is THE departing. In other words, a departing ALL READERS should know about. Would the departing of the church at the rapture be a departing all Paul's readers would know about? Certainly it would be, and the departing of the church in the rapture would SATISFY Paul's theme.
If by "the departing" Paul's intent is the rapture, then who is the "He" that is restraining or preventing the man of sin from being revealed before his time? That would be the Holy Spirit anointing upon each believer. Once the church has departed earth, suddenly there is NO ONE on earth anointed by the Holy Spirit. This is what will allow the man of sin to be revealed, as Paul in his argument shows us in 3b.
Now we know why Paul wrote, "and now you know what is restraining." The answer is, PAUL TOLD HIS READERS - but did it in a way that takes some study. As Peter told us, some of Paul's letters are hard to understand. A beginning reader would not get this. Someone who just reads without study would not get it.
Therefore, I disagree with your take on this passage. I don't believe you have Paul's intended meaning. Take what Peter said to heart: some of Paul's letters are difficult.