The Niners and Cowboys both had great secondaries at times, along with the great Raiders secondaries.
The Seahawks had a great secondary at one point, weren't we one of the highest ranked for getting the most turnovers on of our mid 80's teams ?
All that being said though, the LOB is ranked 1st in my book for a few reasons:
1. They statistically rank near the top, in an era where all the rules favor the offense, even without adjustments. It's hard to precisely adjust exactly how we would've looked 15 years ago as a secondary, but our raw stats are still historic.
2. The are literally the largest, most athletic secondary in NFL history. There's never been a secondary that went 6'4", 6'3", 6'3". Thomas is one of the fastest FS's to play the game, ever. If you let these guys blatantly grab like older secondaries used to, we would literally see sub 100 yard games from offenses. Not every game, but the bottom half of the league wouldn't crack 100 yards on us.
3. Kam Chancellor. I've watched Atwater play. Cherry laid some decent hits. I've watched Ronnie Lott and Kenny Easley is still probably my favorite Seahawk. But if Kam were allowed to head hunt, and the rules that protect players were lifted, Chancellor would literally be killing people. He intimidates entire WR corps right now, I can't imagine how guys would feel going over the middle knowing they could literally be paralyzed by #31.
Add in our physical CB's and Thomas's range and it honestly wouldn't be fair. It's hard to quantify what the LOB's stats would be back in the day, but I KNOW what they could and would do if they played with favorable rules on their side, like teams did 20 years ago.