It is difficult to explain...
Lets pretend it is a car but that the only way to drive faster than 100km/h right now is to use a turbocharger that doubles the fuel burn by just throwing fuel into the engine to generate more power.
New engines are more powerful than older engines but often have the same or better fuel efficiency.
Before if you had the car accelerator pedal hard to the floor you did 100km/h but to go faster you had to engage the turbo, which often more than doubled the fuel consumption so it greatly shortened your driving distance because it burned fuel so rapidly even though you were going 150km/hour you were burning three or four times more fuel so it was just cheaper and easier to go without turbo at 100km/h.
The issue is that if you have a long way to cover and you have others trying to catch you, going faster is actually useful.
Supercruising aircraft are basically supersonic aircraft shaped aircraft... so we are talking formula one cars and not campervans here.
But they have a much more powerful engine... say a V8 which will use more fuel than the current car but only because it generates more power. The fuel burn per horse power per hour might be exactly the same, but the normal car has 80hp without the turbo and maybe 160hp with the turbo, while the V8 might be a 150hp engine without using a turbo and may or may not even have a turbo.
When the normal car wants to move faster than 100km/h it needs to greatly increase its fuel consumption by using its turbocharger.
The V8 burns more fuel than the 80hp engine in the other car but at top speed with the turbo charger on it burns much less.
This suggests a V8 for long range high speed driving is the more efficient. You get there faster and use less fuel than if you had the other car.
The other car makes more sense if it is around the town where it wont go faster than 100km/h anyway so it does not use its turbocharger anyway.
The point is that optimising the PAK DA for supercruising means it wont be a huge high drag aircraft with enormous internal space for large heavy weapons like the father of all bombs, and other large bulky aircraft carried weapons internally including super long range missiles and hypersonic weapons... which is likely the reason they went for subsonic, because the wing can be thicker and higher drag but also high lift for carrying large internal payloads of weapons and fuel.
Subsonic or supersonic is not really important, though the ability to supercruise would have been rather useful.... I suspect the Blackjack will be the super cruising aircraft... it will have four very much improved jet engines with all weapons internal and the low drag of the swing wing design with the wings swept back would be ideal for very high speed super cruising performance.
Flying at supersonic speeds in dry thrust will greatly improve performance for the Blackjack... effectively it will be covering the same distance at twice the speed but not with twice the fuel consumption...
A quick calculation filled with some guesses...
Lets assume cruise speed is mach 0.85 and that is achieved at 70% dry thrust rating.
The dry thrust of the original NK-32 is about 18 tons I think, so 70% of 18 tons is 12,600kgs thrust. If we guess the fuel economy to be 0.72 kg per kghour, that means it burns 9 tons of fuel per hour at that thrust setting.
With an upgraded engine the dry thrust might be 22 tons thrust, but it does not get the penalty of using AB so instead of 2.5kg per kg.hour of fuel burned it has a similar fuel burn (might even be less but lets assume the same).
That means 100% of 22 is 22,000 kgs thrust, which means fuel economy at 0.72kg/kg.h that it burns 15,840kgs of fuel per hour (compared with the original aircraft in a high AB setting that would be burning 55 tons of fuel per hour in AB)
So the supercrusing aircraft does burn almost double the fuel of the non super cruising aircraft... but the supercruising aircraft is travelling twice as fast... so in effect they cover the same amount of ground but the supercruising aircraft does it in half the time.
To achieve the same flight speed performance the non supercruising aircraft would burn its fuel far to rapidly and have nothing like the flight range of the supercruising aircraft.