It was but production was scaled back so much, and the peer enemy aircraft it was needed to defeat still largely don't exist yet. As a result the production lines were shut down and equipment moved to permanent storage and other multirole fighters that are usually less expensive end up doing the F22's job.

In the horrifying event of a US-China war or similar they'd be front line units along with F-35s and other modern fighters, but as is they'll probably end up on service life extension and retired without ever being used much in combat.

The F-15 was sold to other nations and IIRC the majority of its air superiority engagements were with the Israeli air force.

The F-15E and F-15EX (specifically these 'E' variants, aka "Strike Eagles") are multirole strike fighters. They're derived from an air superiority fighter and remain capable of filling that role, but being strike fighters they have a new emphasis on attacking ground targets with precision bombs. It is very similar to the way the F-14, the Navy's old air superiority fighter, was later turned into a strike fighter with the addition of LANTIRN (which the F-15E also got.) In addition to LANTIRN, the F-15E also gained a second seat for a weapon systems officer. Two seat F-15s had previously existed as trainers, but F-15s configured for air superiority normally have a single seat.

The F-22 is foremost an air superiority fighter and was intended to replace all the air superiority F-15s (but not the F-15E Strike Eagles.) However the USAF didn't get enough F-22s so they still have air superiority F-15s and will for some years to come.

It was, but since 22 production has stopped and there are plenty of 15s around, the 35 is often paird up with the 15 for wargaming. Doctrine and training is always dictated by practicalities like which aircraft you have to play with.