The "man of sin/lawlessness" was related to the 1st century.
(2 Th 2:3 Let no one in any way deceive you, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction)
(2 Th 2:4 who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God)
(2 Th 2:5 Do you not remember that while I was still with you, I was telling you these things? )
(2 Th 2:6 And you know what restrains him now, so that in his time he will be revealed)
(2 Th 2:7 For the mystery of lawlessness is already at work; only he who now restrains will do so until he is taken out of the way.)
Paul in the above states the man was alive when the wrote the letter, but was being restrained. The man perished within the regular lifetime of a man 70 odd years or so.
Context and audience relevance is important to understand the above. To posit the man of sin past the 1st century is specious.