Back in the day, how long it took an interceptor to climb to the altitude of incoming enemy aircraft was a critical performance metric. I would think even in todays world world, altitude has great value.
Nobody wants to fight uphill, and everyone wants to fight going downhill.
Altitude=Potential Energy.
I am no expert, but if you are taking off from point X to intercept incoming, if you climb at 70 degrees towards the incoming, you are cutting down the distance the incoming has to travel to reach you. If your calculation tells you that you are only going to be at 10K (with the incoming at 25K) by the time one of you is in weapons range, someone is going to be at a disadvantage.
Now, if you have a missile with 100 mile range, maybe the altitude difference doesn’t matter as much if you are still climbing when the incoming enters that 100 mile radius.
Engaging a penetrating bomber as in the intercept you describe involves different tactics than engaging another fighter in close quarters. You are correct, altitude is an advantage in a dog fight scenario. Trading airspeed for altitude may be an appropriate tactic in some fights depending on the capabilities of the adversary.