So… for those of y’all into tying the lore of G4 into things… a thought.
Seems to me the magic being tied to the crystals, and only working when together, was a peacekeeping fail-safe that was implemented long ago for the future. Seeing how easy it was for the tribes to start warring again before the final battle of FiM and knowing how short societal memories were - and eventually, lessons learned would be forgot and harmony would start to decline - a powerful being (an alicorn perhaps?) tied the world’s magic to the set, with the dire warning that warring and tribes distrusting each other again would lead to magic vanishing for ALL when the set breaks up. Obviously it didn’t work, but that sort of deterrent would certainly be an effective way of at least attempting to keep the peace long after you’re gone.
After all, if you threaten to take away ALL the toys if kids don’t share, with luck they will get very good at self-enforcing sharing…