Unless you have your own car. Most - not all, but most - who drive to WDW probably skip the hell that is a Disney Bus. I know when I stay on property, I never, EVER use a bus. I did once, and I certainly learned my lesson: never again!
As for your directions, that IS how the detour was directing guests, but your directions are only half complete. Turning onto Osceola isn't gonna get you there - it's just going to take you to DAK or DAK lodge. After you turn onto Osceola, you have to then take the exit for World Drive heading south. You keep going until you're off property (about half a mile from I4), and then there's a blinking traffic light. Now, you can just do a U-turn and it'll get you to the main entrance of Studios in about a mile. But the detour wasn't directing people that way (probably because it isn't wise to have thousands of cars trying to make a U-turn at a blinking traffic light). The detour, instead, was having people turn right at the light, head down that back road for a couple of miles till they hit 192, onto which they'd turn right, then they'd drive about another mile to take the exit ramp for Disney, bringing them right back onto World Drive, just heading north this time, at which point they could just follow the signs for the main toll booth of Studios. The only way to get onto I4 to head west would be to catch I4 off of 536, which is accessible for some resorts. But many would have to go by way of Lake Buena Vista drive to get there, and with the amount of traffic heading to DD, I'm sure they didn't want to add to that headache if they didn't have to.
