Except it won't?
If the B-21 doesn't reach the number of production needed, it's more likely the US simply won't have a bomber leg of the triad anymore, rather than soldier on with an old airplane, though. The reason for 100 B-21s minimum is obvious: that replaces the 76 B-52Hs and the 21 20 B-2s in the GSC's order of battle, with spares.
B-52 will be gone in 20 to 30 years and likely sooner than later, whether it wants to be or not. Even if it's still there on paper it may very well end up being hollowed out to pay for new aircraft. If the USAF has to, I suspect it will cut into B-52 operational squadrons, and kill the re-engine program (or at least heavily foot drag it, like the A-10 rewinging program), to pay for B-21 and bring their squadrons online. Shuffling funding around from A-10s or B-52s to fund JSF or B-21 is pretty trivial.
The new bomber just needs to make the hop from testing to production, which it seems to be doing rather better than B-2, which is sort of to be expected. If B-21 doesn't shake out at the end of the day, then I think the US simply won't have a bomber force in the future. It's not a terrible outcome. Both Britain and France lack strategic bombers and they do just fine as regional powers go, and America could yet go the same way in the future.
Submarines are more survivable and silos have better readiness rates, after all. With those two alone, America will still have a strategic ladder to climb to the top of its nuclear awning and shout into the escalation vortex, even if it's not a three-legged barstool to slap someone over the back with in the Thermonuclear Bar Brawl 20XX, or whatever overwrought analogy of the day is being used.