Well, I myself am still new to Manjaro, albeit that I'm not new to GNU/Linux as a whole ─ I've been running GNU/Linux exclusively for over 20 years now. So I hope my comments won't be regarded as presumptuous, but off the cuff, I already see three problems with what you're suggesting ─ there might even be more.
Forcing a narrowing-down of the amount of distributions being offered is an artificial thing, because that's not how the GNU/Linux ecosystem works. GNU/Linux exists because it entails and embodies freedom. By stating up front that only a limited number of distros should exist and that the developers should concentrate on one of the distros in that limited offer, you are depriving people of their freedom.
It would never work, and the licensing of Free & Open Source Software doesn't allow for that. Quite the contrary, even: if you were to enforce something like this, then you are only going to create strife, and you will be driving people away from your distribution. The antics of Caldera Systems (alias The SCO Group) spring to mind.
As much as I imagine that Manjaro would be welcoming new developers, there is always a risk, if developers from another distribution were to join the team, that they would compromise the goals, philosophy and identity of Manjaro as a distribution. Because in the end, other distributions exist because people have their own ideas about how things should be done, or what target audience they are aiming for, and so if those developers were to start working on Manjaro, then they might bring their own vision and standards with them, and those may not be compatible with Manjaro's vision. In the end, that's how forks are born, and I've witnessed the birth of quite a few of them...:
** PCLinuxOS forked from Mandriva when Bill Reynolds was laid off
** Mageia forked from Mandriva when a number of other developers were laid off
** Mandriva proper became OpenMandriva
** Devuan forked from Debian over the adoption of
** CentOS forked from RedHat but is now back under RedHat's control
** ... et al, plus similar stories with the BSDs, and what used to be OpenSolaris
That has so far always been how the GNU/Linux ecosystem works. Only the strong survive. Now, I'm not saying that this is necessarily a good thing ─ especially not if the concept of "being strong" boils down to "having the financial resources" ─ but the way I see it, the GNU/Linux ecosystem is somewhat of a meritocracy.
Quality is its own best advocate, and the best distributions tend to survive the longest ─ at least, if there are no corporate shenanigans involved, as has already been the downfall of a few distributions in the past, and the same is true for bureaucracy and ego manifestations. That's what almost killed off Gentoo a number of years ago, and from what I've seen by monitoring their developer mailing list ─ which I'm not monitoring anymore now ─ they still haven't completely been able to outgrow either the bureaucracy or the ego manifestations. And sooner or later, it's going to come biting them in the behind again.
We all have our own preferences, which is why we all pick the distribution that we like the most, and the desktop environment we like the most, and we install the system in the manner we prefer, and we install the software packages we like the most, and so on.
Every individual is different, and the GNU/Linux ecosystem provides something for everyone of us. And for us here at the forum, that's Manjaro. But an Arch proper user might look down on Manjaro as being too user-friendly ─ read: not l33t enough for their taste ─and might prefer sticking with Arch proper. Or there may be people who like things more dumbed down and who hate reading wikis and
man pages. They want a turnkey distribution, very much akin to the consumerist operating system offers from Redmond and Cupertino.
Again, quality is its own best advocate. Like I said, I've been exclusively using GNU/Linux for 20+ years now, so I've seen quite a bit of the GNU/Linux ecosystem. And in all of that time, I had never seen a distro so well put-together and so clean as Manjaro. If Manjaro can charm a 20+-year GNU/Linux veteran like me upon the first encounter, then it can charm a hell of a lot more people.
And that is why I believe ─ and hope ─ that Manjaro will always continue to exist. But other distributions that want to survive will have to be just as good, or damn well near it. It is after all easier for us penguinistas to walk away from a bad distribution (or a distribution you just don't like) than it is for many people to walk away from Microsoft Windows or Apple macOS, given that it may void the warranty on their hardware.
So my personal opinion is that it's probably best to just leave things be and let them sort themselves out. The good distros will prevail and grow a larger user base, but there's still always a distro that covers one's needs, and if those distros garner success, then power to them.
In the end, it's all about freedom.