Different users have different requirements, but it makes sense to make variants of engines that are new rather than engines that are old except for upgrades.
Very simply the example you mention, the D-30, PS-90A, and PD-14 are basically very similar engines, each with their own advantages and features.
The D30 is reliable and in service but not the most fuel efficient or clean running engine available, but adding new features developed for newer engine designs there is scope to improve its performance in a few areas to make it still a useful engine.
On the Il-76 its main problem in the past for the VDV as a customer was it was too fast... the air buffet jumping out of an Il-76 was more than they would prefer, but it could carry all their equipment like armour etc.
The An-70 was a replacement though it was an odd duck but being a turboprop it could fly safely at lower flight speeds so it was easier and safer to jump out of and it meant than instead of being spread over 8-10km, the payload might be spread over 4-6km which means the units delivered could form back up and get moving much faster... but then the illegal US coup happened and the An-70 died a natural death because there are no other customers like the VDV in the west and even if there were they had the A-400M that they would have been forced to buy.
The main point is that for the VDV to have a turboprop transport of lower flight speed to deliver their troops and armour it is simply not worth developing a brand new aircraft completely from scratch AGAIN... especially as they would also need a suitable engine too, but as they are now putting the Il-476 into production it certainly makes sense to develop a model or variant with a turboprop engine that could perform the role the VDV want executed... it would basically mean developing a new turboprop or propfan variant of a new engine... I would say it makes sense to develop the newest engine available which would be the PD-14 in a turboprop or propfan variant.
The PS-90A offers better thrust, lower noise and emissions, and better fuel efficiency, but it is not cheap.
The PD-14 should offer even better performance but a propfan model might result in lower speed operation, which for most customers is not so useful on a strategic transport like an Il-476.
For a medium range transport like the Il-276, the lower speed and costs of a propfan might be appealing so a twin engined Il-276 with PD-14 based propfans might be interesting too, but certainly the VDV would prefer a larger heavier aircraft that can carry armour as well as troops for dropping.