Dropouts without chain tensioning slot were simply unheard of at the time and if you happen to be the inventor of the quick release it's not really surprising that you make it a key element of the shifting process.
I was wondering for a while about the purpose of the teeth on the axle/dropout, because clearly with bearings between cogs and axle there's no way to force the wheel forwards pulling on the chain (you force the wheel backwards for higher tension by applying a little braking force), but the teeth on the axle are what makes the movement in the dropout symmetric, so your wheel will remain aligned to the frame center line. Really clever!
I was wondering for a while about the purpose of the teeth on the axle/dropout, because clearly with bearings between cogs and axle there's no way to force the wheel forwards pulling on the chain (you force the wheel backwards for higher tension by applying a little braking force), but the teeth on the axle are what makes the movement in the dropout symmetric, so your wheel will remain aligned to the frame center line. Really clever!