I've seen a few threads recently with people mentioning the update size size vs their bandwidth. Then I've also seen a few users recommend switching to a non rolling release distro like Ubuntu or Mint.
I can understand that a rolling release may not be for everyone, but we have to think about why people initially switch to Manjaro. For many it is access to the AUR. All the software you would ever want, and then some, all in one place. Non of that having to add a PPA after first searching the web for a software that might fit your needs. Non of the worries about malicious repositories advertised on some random warez site.
How about this, create a fourth "frozen" branch alongside stable and testing. This branch could simply be a snapshot of the final stable within a year. The only updates to this branch should be done on a yearly/bi-yearly schedule, and possibly the inclusion of serious security patches.