Why should anyone forge 5* armor/shields to level 10? It's even debatable whether you should level to 9, 8, 7 or 6, but for simplicity I'll stick to just talking about level 10.
I have a Dread Skelly Shield and Mask (both 5* and fully heated at level 9). The amount of Radiant Fire Crystals that I could potentially pump into these is mind boggling, and the payoff is immeasurably small. I understand that with weapons you get a bump to charge time reduction at level 10, but with armor and shields there is no such bump. You just get the typical increase in standard damage type resistance - for the Skelly line it's a bump to normal and shadow damage - but I can't even see the dim part of the bar that will be awarded when I forge to level 10.
For armor the last significant stat bump is an added health bar at level 5. For shields there is no significant stat bump, just gradual increases to damage resistance.
So my question is did OOO intend for 5* level 10 armor/shields to be a status symbol? Or is there some valuable benefit that's not discussed anywhere I've looked?
With the introduction of fire crystals and the amazingly high jump in requirements past level 7 (as compared to the higher frequency of orb drops), having gear that isn't level 10 is now an actual consideration when choosing where to apply your resources.
Basically, your gear gets slightly more optimal at the higher levels of heat. That's it. Eventually people are going to want their favorite gear to be entirely optimal (this is why we pay out the nose for UVs which offer marginal benefits), but for those who are choosing where to apply their fire crystals and have to pick between one piece of gear and another, getting things to level 10 is a lot less attractive.