> THz radar

I've got your THz right here (shines laser pointer at sensor)

You'll see puff pieces from radar manufacturers saying "we spotted your best stealth plane, game's up," but they tend to leave out critical details like how more sensitive radars tend to be more vulnerable to jamming. Detecting a stealth aircraft in peacetime is a very different prospect from detecting a stealth aircraft in a hostile EW environment, which is itself different from an outright hostile environment where every radar pulse is a targeting opportunity for an anti-radiation missile.

Also, it's not black and white: range of detection matters. Shrink it enough and either your opponent has to put up more radars or you get the opportunity to slip through. Retro-reflective stealth is defeated by networked radar, but then the opposing forces have to maintain a network and you can try to disrupt that network. Even if you can't disrupt the network, it doesn't fit on a missile, so now the missile either can't target you or needs network communication to target you, and now you can try to go after the missile's network, which is wireless and even easier to mess with.

Stealth isn't necessarily an overriding factor these days, but it definitely gives you options and puts pressure on the opposition.

AFAIK it's the opposite. Low frequency early warning radars are able to detect the vague position of stealth fighter jets (like the F-35) because these jets need a vertical stabilizer to be able to do combat maneuvers. Knowing the vague position is good enough to route intercepting aircraft towards the threat but in theory you will be unable to target these jets with AA missiles because targeting radars can't see these jets until they're very very close.

Flying wing designs like the B-2 and B-21 don't have this issue and are mostly invisible to early warning and targeting radars.

Cynical take: jobs program for the senators district.

Non-cynical take: Most modern aircraft are equipped with networking/avionics that allow information to be shared across multiple battle domains (army on land, air force, maybe space who knows). A stealth-ish (not completely invisible but still tough to track) aircraft that can perform this role as well as carry nukes might be useful in coming conflicts.

It just needs to be invisible enough to have the planes be able to destroy targets before the enemy (a) detects and determines what you are and (b) launches a set of missiles to destroy you.

If stealth fails at (a) but buys you enough time to get out before (b) then it's still a successful technology.

Hiding in the THz is the easiest; paint it black and fly at night.

Lower THz, e.g. IR, is attenuated by the atmosphere. Useful but limited, and stealth planes are designed to make it harder (e.g. by mixing hot exhaust with cooler air above the wing, where the hot engine cannot easily be seen from the ground.)

"Stealth" means "as stealth as we can be", not "100% invisible".

> hiding from modern THz radar is impossible

Seeing a plane and targeting it as separate beasts.

The radars of WWII could see these aircraft but with extremely low resolution. What THz phased array radars are in service right now?