There are some areas where certain achievement is valid for specific periods - like GATE, NET, etc. when achiever has limited period to bank on the achievement. There are certain other areas where the achievement/event is seen more permanent - IAS, Chess GM, Degrees (BE/BTech/MBBS/CA/LLB etc.). It is well understood that in case of permanent conferrng of honour too, quality may change (increase/decrease) later. Chess GM's rating may fall, skill of a degree-owner may get blunt due to lack of use, etc.
Since PhD (or DLit, DSc etc) is an honour and not a gateway like GATE/NET, it should remain permanent. However, to ensure that a person does not bank on a single achievement throughout life, other criteria may be set for a particular career. e.g. do this or that withi next 10-12 years.
