Keep in mind, with each and every pop added to the capital the new reborn is easier to build.
Also, it was a duel map... I couldn't fit that many cities in.
Only to a point. It's not that hard to get down to three or four turns even with a happy-cap, and only at most the first 24 pop can work the map. Then you're limited to Engineers or Citizens. (Maybe priests.)
Citizens are awful, so you need either Glory or one of the other unlimited Engineer civics. Or you can use an unlimited priest civic, if you have a high level Altar. So you need more late game techs, and by that time you might just use Unyielding Order.
Also, you can't use more than 345
per turn. Factor in production modifiers, lets say +125% (Godking, half the luxuries, Forge, Artisan shop, one master building), that's 154 raw. Let's say 4 each from the 24 first = 96. If the specialists add 2 each at pop 53 you will be building one Reborn per turn, extra pop is wasted. Better specialists or better production modifier pushes that lower.A 150 pop capital isn't worthless, but it gets hit hard by the rule of diminishing returns. The Tower of Complacency doesn't need to be blocked, because the benefit it provides is marginal at best. Scions have huge happy caps even without it and can effortlessly found new cities.
On a duel map, the production needed to build all those Reborn must have been several times what would have been necessary to conquer the world three times over.
So Tower of Complacency is blocked because it can be used with a gimmicky strategy? One that is still available through Unyielding Order? Why don't we block Spring because it can be used to spell dirty words in deserts?
a pop is too easy. Building them as Reborn is too hard for it to be a viable strategy. Capturing them as slaves just might be the just right.
is your main concern, adding a -
It provides a promotion to all your existing units that guarantees they will create a slave from combat. Lasts 15 turns. 
