The slot is well protected behind the rear plate. In a later, retrograde, step Leica made a series, designed M (mucked it up) where most of the back hinged up just to get at the slot!!!
They also made the M series with a bayonet fitting lens, just like the new Sony, you know the one that lets the light in without going through the lens. A screw fit lens will never do that.
The battery never needs replacing BTW, another technological advance they dropped when they found they could sell new batteries at the price of compact camera.
Also they tried the "upgrade for life" on the M8 series, what a joke, the screw models have always had that and they actually did it not promise it and never deliver.
(Thanks for the chance to rant 😀 )