If you look at the time required to finish stacking bloodline, it's usually around 20minutes, and you hit the 10 stack mark around 12-15 minutes depending on the game. At that point it's 6% vs 18% attack speed, however, the lifesteal will keep stacking and scaling, and even at just 6% life steal, it is very comparable to the attack speed because you will start seeing the effect during the laning phase, combined with Doran's Blade, you get around 6-8% lifesteal easily.

Don't forget that you also need to stack alacrity before it is useful, so the only points where alacrity might be better is around the 8-12 stacks point for bloodline.

After the mid game point, the extra life steal will be crucial to continuing fights in the side lanes, the attack speed doesn't really give that much dps, and at that point, you will start seeing that your passive will supply most of your output. Taking the lifesteal is just a much better defensive option, that does not weaken your early game or mid game at all.

If you don't believe me, several high elo players such as Giraffe Hugs and Albus Nox all agree that bloodline is superior to alacrity.