A tale of two planes, the B-52 and the T-38. The B-52 was built to carry dumb A and H bombs into hostile, non-SAM airspace, a long way away and became operational in 1952. To do this it had to be big and powerful. This made the airframe adaptable. As enemy defense improved it had the growth to carry countermeasures. Weight increases was off set by substantial power increase of 42% as engines developed. Avionics advanced allowing terrain following. Because it was big and powerful it could be adapted to drop convention bombs, lots of them. Also, new weapons like SRAM, cruise missile, and JDAMs extend the usefulness of the big bomber. But it did become obsolete in its original mission, flying downtown enemy cities and dropping bombs, first by the FB-111, then the B-1 (maybe), and finally by the B-2. The B-52 remained effective because it was adaptable to different missions.
The T-38, which became operational in 1961 was designed to a specific task, provide advanced capability to AF pilots who are transitioning to newly evolving high performance jet fighters. While marginally adaptable with a slight increase in power and upgrading avionics, the T-38 still outstandingly performs the same mission over 50 years later.
These two aircraft, both with over 50 years front line operations maintain relevancy for two different reasons, the B-52, an excellent design, which was adaptable to a changing battlefield and missions, and the T-38, also an excellent design, which performs its rather straight forward unchanging mission in such a manner that there has been little competition to replace it.