That was in the 1970s, over half a century ago, and under completely different military doctrines and a different battle space. The US sent in 200 B-52 bombers during that 10 day period to conduct stand in bombings (flying directly over the targets), and between 16-34 were shot down by North Vietnam, the vast majority by S-75 (NATO name: SA-2) Surface to Air Missile (SAM) systems.

Since then, air defence systems have gotten more advanced than the S-75, and bombing doctrine has changed as result. Four US B-1 bombers are not going to be conducting stand in bombings over North Korea, China or Russia, just as Russian Tu-160 bombers are not conducting stand in bombings over Ukraine.

The value of these aircraft to a modern air force is their massive payload capacity of stand off weapons, mainly cruise missiles, for stand off strikes carried out far away from enemy air defence systems and territory. We see this with the Russian Tu-95M and Tu-160 bombers against Ukraine, they launch Kh-101 cruise missiles targeted at Ukraine from the Caspian sea, or from within Russia itself. A single B-1 can carry 24 JASSM or LRASM stealth subsonic cruise missiles in it's internal weapons bays (the most of any aircraft in the USAF), and a further 12 externally if the external hardpoints are fitted (most B-1s don't have these fitted anymore). Most fighter aircraft can only carry a handful of these, an F-35 can carry 2 on external hardpoints, an F-15E can carry a maximum of 5. JASSM-ER and LRASM cruise missiles themselves have a range of around 1000km/600mi. So if B-1s were to fire JASSMs at land targets, or LRASMs at ships, they would do so from well outside of the opponent's ground based air defence range, and with their own fighter escorts and electronic warfare aircraft to prevent interception of the bomber by opposing fighter aircraft.

The challenge for air defence networks (including ground based air defence and fighter aircraft, and AWACS aircraft) in a modern war won't be to shoot down the bombers themselves, but to detect and intercept the cruise missiles before they hit their targets. As we've seen in the war in Ukraine, this is a difficult task to accomplish, cruise missile interception rates for Russian missiles range from 25-66%, depending on the type of cruise missile fired and how difficult it is to detect. The Kh-69, Russia's stealthiest cruise missile, was jumbled up in the data with the less stealthy Kh-59 at a 25% interception rate. So intercepting these weapons is difficult. Four B-1s is a maximum total volley of 96 JASSMs at ground targets, or 96 LRASMs at ships, per sortie. Even if a sizeable amount of these cruise missiles are intercepted, those that slip through can still do a ton of damage, these are precision guided weapons, and it's unrealistic to be able to intercept all of them.