Warning: spoilers ahead for The Death of Doctor Strange #1!
While his killer is as of yet unknown, power of the Sorcerer Supreme is like a force of nature, protecting the Earth from all sorts of threats and interdimensional dangers that are continually trying to invade and destroy Earth. If not for Doctor Strange, it's been revealed that the world would have fallen to ruins a long time ago. Now that he's gone, the planet seems to be in more danger than it's ever been before, proving how important Strange truly was.
Before his death in The Death of Doctor Strange #1 from writer Jed McKay and artist Lee Garbett, Doctor Strange had become privy to rumblings across the dimensions, hinting that something big was about to occur. However, the Sorcerer Supreme continued to go about his duties to keep the world safe while also teaching at the Strange Academy, his school where the next generation of magicians are learning to harness and control their abilities. During his class, Strange breaks down how crucial the role of the Sorcerer Supreme is to the world's survival, likening the protective master spell he casts to the ozone layer or Earth's magnetic field. Just like they respectively protect the world against UV radiation and solar winds, Strange's magical barrier protects the Earth from all sorts of interdimensional threats (so long as he's alive to maintain the spell.)
As a result, the Earth immediately falls into chaos as soon as Doctor Strange is killed by an unknown assassin. The moment the Sorcerer Supreme dies is felt around the world, as the entire plane of reality has now shifted out of balance. Interdimensional attacks begin almost immediately, causing the Avengers to scramble to multiple points across the globe to somehow handle the dangers that typically would have been stopped by Strange himself. As a result, it's become pretty clear that Doctor Strange was one of the most important heroes the Marvel Universe had.
Thankfully, it seems as though Doctor Strange had a failsafe where a younger version of himself would be summoned to the present in the event of his death, though that seems to be a temporary solution. A new Sorcerer Supreme will need to be found and approved by the Vishanti as soon as possible considering how much the world is already at risk without Strange. According to the Strange Academy's librarian Zelma, a handful of potentially world-ending threats have already breached and come to Earth without the protective magical barrier cast by the Sorcerer Supreme.
It looks as though the death of the Master of the Mystic Arts will open many heroes' eyes to just how important the Sorcerer Supreme is now that Strange is gone. To both Marvel fans and the Avengers, Doctor Strange can sometimes seem removed from the day-to-day process of saving the world, stepping in for major emergencies or magical consultations, but that's only because he's such a consummate professional, often using his ability to travel time and space to go on epic adventures no-one else even witnesses. Like Iron Man before him, Doctor Strange has often been treated as a second-tier hero, but his new prominence in the MCU and the consequences of his death show he is now - and always should have been - on the A-list.
