What are the pros and cons of high service ceilings for fighter jets?
If your question is about advantages and disadvantages of the high ceiling:

Advantages: 

-For military aircraft, reaching a high ceiling gives a tactical advantage in air combat and makes it possible to escape from surface-to-air defense . For civil aircraft, it is above all a question of achieving the best efficiency of the engines or of being able to take advantage of the air currents . For both civilian and military helicopters a relatively high ceiling allows intervention in mountainous regions.

-During the Cold War the race for the altitude record was part of the propaganda war between the two blocks. It is however necessary to distinguish the maximum altitude reached during records, not supported, and the operational ceiling of an aircraft, given for standard mission (military) or operating (civil) conditions, and which is the maximum altitude to which the plane can stay in a sustained manner. For example, the absolute altitude record for a conventional jet plane, held by the Ye-266M (one of the prototypes of the MiG-25 ) is 37,650  m , but this same plane has a ceiling of "only" 24 400  m . In the same vein, the Lockheed NF-104A, a demilitarized version of the F-104A Starfighter used by NASA to train the pilots of X-15 , could climb to altitudes higher than 36 600 meters thanks to its additional rocket engine, but its actual ceiling was only about 15,000  m .

Research continues to push the limits reached during this period, with aircraft such as the X-43 and its supersonic ramjet .
Disadvantages:

but nowadays, missiles have evolved and fly faster, higher .... now it's not a race for altitude.
now you have to be stealthy, have good self-protection systems.

it's time for the hypersonic era!

"The first Russian Avangard hypersonic missile regiment, capable of spinning at more than 33,000 km / h and piercing any existing missile shield, has officially entered service."china have same hypersonic missiles