Most simulations place the infection rate closer to half, and as high as 70%. The longer it is until the vaccine is designed, tested, produced, and then finally distributed...the wider the spread. The US in the modern era will never successfully implement a wide scale quarantine, so there's going to be new cases literally every minute of every hour. Testing will never work fast enough to contain it, and with half the carriers being asymptomatic there's no way to single out people to lock down.

It's a truly worst-case scenario, the 1%-5% fatality rate (with nearly 0% for under age 50 with no underlying conditions) means that if 70% of the world gets infected you're left with around 50,000,000-250,000,000 dead - 2-10 mil in the US alone. The vast majority over the age of 65. There's about 600,000,000 people over the age of 65 in the world, so this will quite possibly kill 25% of them if it's allowed to spread unimpeded.