The lifeline is obviously much shorter once it is reanimated. We are talking six months in perfect conditions, and a week in the worst conditions. And you are right the virus would be slow to reproduce, but it is easy to contract. And the virus itself isn't lethal. Like it was also said there is lots of bacteria that feed on dead bodies, and whenever a zombie bites someone that bacteria is passed one causing infection. A person could technically survive a zombie attack with proper care and antibiotics, and live to a ripe old age. But once they doe they will be reanimated. That is one thing that makes it devastating. We could be under attack. Millions of zombies. But human band together and repair the world and start anew. But fifty years later when a few people that were bitten unnoticed die, the process could potentially start over.
Something else, is that animals cab also be reanimated. And since animals are nothing but primal instincts, they could be controlled much easier by the virus allowing them to movie at almost a normal speed. The problem is though is that if an animal gets close enough to get bitten, it is most likely going to be eaten, and since a majority of animals are smaller than us there would be hardly Any remains left to be reanimated. But this could still propose a very large and terrifying threat. Imagine a dozen or so zombies limping toward you, and in the middle of them is a grizzly bear? Slightly horrific. And the problem about staying in your house because you think it is fortified is that alot of the times you won't be ready for the zombie attack and won't have enough supplies to last. Do you have enough food and water to last 2-6 months? And another is that zombies don't have nerves. They will scratch and pound on your house until the skin is stripped away and it's just bone. And then they will continue until the hand has been worn down and it's just arm. Even sturdy houses might not be able to handle the non stop constant hamming.