I think you'd find that most LSM folks don't believe the competitor should be removed. You can find evidence of Casey saying that there are reasons to use SELinux but he wanted a more light weight module. I've never seen the TOMOYO guys say that SELinux shouldn't be there or even better that AppArmor should never be included (as they are both pathname based). Because there is the desire for distributions to ship multiple LSMs we included the ability to choose at kernel boot time which one you would like to use. Ubuntu uses this functionality to ship an AppArmor and SELinux enabled kernel. To move from one to the other it consists of installing the correct packages and either setting apparmor=1 or selinux=1 on your kernel entry in grub. The Tomoyo guys have said it would be nice to run Tomoyo and SELinux at the same time but I can't really see a good use case for doing that.
The problem with unwinding the allocation and deallocation stacks is mostly with deallocation. It means that you need to ensure that the entire stack can dealloc something before you start going down the stack otherwise you may have two layers removing their security information from an inode for example before it hits a layer that fails. Now you need to repopulate everything at that point above the stack. I'm not saying its not doable but that's just one tricky case out of many you'll run across.
Also another example would be what do we do with labeled protocols now like Labeled-IPSec and Labeled-NFSv4? What do I send across the wire when I support both smack and selinux labels? What if tomoyo wants to send the label on their process as well? Do we make the hook to get all relevant security information return this massive blob of data for a number of LSMs that might not be needed?
There are a lot of things that need to be thought about rather than just saying hey this should exist. I've payed attention to the stackable LSM conversations and no one has actually proposed a solution to this yet. Everyone just seems to say it needs to be done and walks away.