Yes, there is a certain amount of competition for users that way, but they're not really on the same turf. Apart from the Peg2 they don't run on the same hardware, and neither set of developers is going to change that. New (returning) users have the choice of which OS they want to use, but it's not an equal comparison since there's a vast rift between the hardware bases. So they can choose to spend a heap of money on new hardware and AmigaOS4, or far less money on 2nd hand hardware and MorphOS, or less money again on AROS. Different people in different situations are going to go for different approaches depending on what they need or want, and seeing as there's so little overlap between the different systems, I'm sure most potential customers would end up falling into one or other system almost automatically.
Perhaps those OS4 users you say are retreating into a shell are sick and tired of being told that they made the wrong choice, that they've wasted their money, and so on. What's the point in that? I'm well aware of the differences between MOS and OS4, and I get quite fed up with it. I enjoy my hobby and don't want to be constantly given reasons again and again why I shouldn't be partaking in the way I do, why doing it the MOS way is better, cheaper and faster. I don't care, though I wouldn't consider myself having retreated into a shell. I'm just ignoring the criticisms that float my direction so I can enjoy my hobby as *I* want.