Isn't comparatively cheap IRST/FLIR proving a thorn in the side of of this expensive stealth technology?
Nope, passive infrared devices can't see much too farther than visual range in humid and/or dusty air. Active infrared radar has less range than microwave radar in any atmospheric conditions. ( Microwave wavelength radar -- that which is commonly called "radar." )
Radar stealth affords low observability at distances beyond passive electro-optical sensor or eyeball range. Radar stealth is what one needs for successful 21st century air to air bvr combat.
Let me ask this rhetorical question: why do longe range air interdiction missiles use radar guidance instead of IRST/FLIR?