Apple is greedy not stupid. They don't want third party app stores because it would cause competition and interfere with their almost entirely vertically integrated consumer milking machine... they don't care about malware, they care about control, controlling the user, to make sure they spend as much money as possible with Apple.
Notice that they have been increasingly swapping out as much third party as possible so they can take all of the profit, they "produce" film and tv shows and sell those tv shows on a device they make with a payment processing method they own... lots of layers of profit, it goes well beyond monopoly, they are destroying the concept of a free market. They do similar things with hardware and tech, buying out or forcing businesses into positions where they have to sell to Apple out or get pushed out.