This is why both platforms exist. There seems to be this strange idea that the F35 must be compared against the abilities of fighter in a direct 1v1 dogfight scenario, despite that this situation would almost never happen. Modern air-to-air engagements are not squads of fighters dueling one another, but instead entire fleets of aircraft with differing roles and responsibilities working together in a coordinated fashion. AWACs, electronic warfare planes, missile trucks (like the F15EX), and fighters that can get in close all working together. This is environment the F35 was designed for. The fleet level data-link and sensor fusion capabilities of the F-35 are it's main feature, as they augment the capabilities of the entire fleet. Then fact that F15EX can act as a better standalone interceptor is not really relevant, since it would almost never perform this role unsupported. An F35 and F15EX operating in tandem would be much more effective than either operating alone, since the F35 could enter areas covered by opposing fighters / ground fire, and feed sensor information to the F15EX to take out threats outside of its own sensors range.

This is why both platforms were designed to be used together as a team. F-35 provides sensor fusion and the F-15 is the missile truck. That also allows the F-35 to continue to evade detection, since launching attacks tends to get you noticed by the enemy.

Modern air combat can't be measured in direct comparison like this, or even thought of in terms of "we should be using X instead of Y"

The F-35 costs are also going down relative to it's peer group as the export market has grown significantly. The F-35s issue is that it's misunderstood, not that it doesn't meet expectations.

Isn’t the F22 the successor to the F15, being air superiority fighters?

The f15 is going to be shoot down 60km from target by s400s, the f35 can get close enough to drop an harm on them at a speed they cannot intercept, so there's that.

Also the f15 has speed or range, and if you need one it reduces the other.

You try flying an F-15 with 12 to 24 Slammers (and bags since you'll have so much drag. That's going to fly like a pig. Just because Boeing does some demo of it to sell more airframes doesn't mean it's going to be used that way.