The "running away" concept is highly overrated here, especially when the B-21 is going to be cruising at around M0.9-0.95 and the J-36 would cross transonic and cruise there M1.2 to M1.3 at best. It might be able to do M1.6-1.7, but at that speed, it will become a low range garage queen that needs too much maintenance for high sortie rates. If you are basing these assumptions on the false belief that the J-36 will be a M2-3+ aircraft which seems to be a common trend on this thread, then I'm sorry to disappoint you, but that will never happen. The J-36 does not display the important fundamentals of a high speed aircraft. It is not a low aspect ratio or a high fineness ratio aircraft, length equal to or less than the wingspan, has three individual intakes, one likely to be a DSI, no vertical stabs or ventral fins, a very thick fuselage starting right from the nose, a flying wing/bwb, and its parasitic drag will climb too fast for it to reach these speeds.
Way too many people here seem to think that the three engine configuration will just magically propel the J-36 to higher speeds, underestimating drag as some minor opposing force that can simply be overcome with extra thrust. Even bpr is being disregarded and it is being assumed that 3xWS-15s will do the job. If adding thrust was the main limiting factor, all planes today would be travelling at high supersonic speeds.
Anyone who doubts this should spend some time researching high speed planes such as the XB-70, SR-71, Concord, MiG-31, Tu-144, X-15 etc.. You will quickly come to the realization that each of these designs give so much away for a drop of return in speed gain. The XB-70 has 6 turbojets (no bp), an aspect ratio of 1.751 that is then trimmed down to 0.766 with the folding tips, a ridiculously high fineness ratio as you can see from its shape and length, a flush cockpit, has titanium all over its leading edges and high temperature areas, just like all other high Mach planes. Despite all these features, the plane only does M3 and needs a mind blowing 140 tons of fuel for a combat range of less than 7000km.
I have even seen comments regarding how the J-36 looks like an efficient glider, implying that it generates lift efficiently and so has lower drag because of that and because of the fact that it has no vertical bodies. Have you ever seen a glider flying at Mach 1+? Take a quick look at lift induced drag equations. Lift induced drag is directly proportional to the square of lift, inversely proportional to the airspeed velocity squared and the wingspan squared. The coefficient of lift induced drag is inversely proportional to the square of the velocity. The coefficient of lift induced drag goes down as the angle of attack decreases with the increase of speeds. This is why lift induced drag does not matter for high speed aircraft. It goes down as the plane goes faster. Where the real problems start is with parasitic drag, because whereas lift induced drag is inversely proportional to the velocity squared, other types of drag are directly proportional to the velocity squared, hence this very famous drag photo that you see in drag related books and publications:
Once we enter the transonic/supersonic regions, we start to run into compressibility drag related issues and shock waves, and those are a whole nother story. This is why all these planes share the same design philosophy of thin long fuselages and noses with tiny, thin and highly swept wings that are pushed as far back as possible.
The only plane that stands out in this crowd in terms of long and thin fuselages, is the Mig-31, which tries really hard to conform to these rules, but it is too small and needs some maneuverability and pitching moment to intercept and carry armament. Despite that, it still has 3.5 tons of titanium, razor sharp swept wings and horizontal stabs, a small wingspan and very large highly swept vertical stabs, and its wings are so swept and small, it needs wing fences to land and take off. It has a combat range of only 720km at around M2.35. How far do you reckon it would go at its max M2.8, 300km? That is only enough for it to climb up, reach that speed, then glide back down to land.
Neither the B-21 nor the J-36 will ever see combat in highly contested airspace and both will be using standoff weapons launched from safe distances. If either were to ever be spotted, locked on, and fired upon with missiles that travel at at least M5+, they are both going to have to rely on literally anything but running away.
In the case of the J-36, it will have a significantly higher chance of draining incoming missile energies, not due to higher speeds, but due to its high thrust and ability to maneuver/dive quickly compared to the high moment of inertia and low pitching moment B-21, but the B-21 is strategic bomber with far better VLO features, and will be able to carry larger payloads in terms of mass and volume. These payloads can still have longer even if their initial launch velocities provided to them by their B-21 were lower than the J-36. Each of these two bring their own game to the table, but neither are disadvantaged in their own domains.
