You just typed out the FOMO part yourself. If it is limited - it has FOMO, no matter how much or how little.
Warframe's always positioned itself as a no-FOMO allowed kind of game. From NW, to prime resurgence being core to the experience of "If not now you will have more chances later". These Heirloom skins go against what Warframe's been for a decade. It's like a plothole, lol.
What are we 'funding' here, too? Prime releases exist. Game's been out for a decade, if this is to feed Soulframe, make a Soulframe founder's pack and make us ride the wave of "hopefully that works out".