This is why its canon.
Warning.
If John had been the one to go, Sherlock would have lapsed into self-destruction. First with cases, the risks seem insignificant. Every hand plays close to the bone… and soon after, drugs. Everything faster and harder then before. Keeping the pain at bay in double, triple doses. Until something has to break.
Just as someone untethered.
So John, without Sherlock, sinks. In stoicism and silence he falls slowly, almost forgotten. No one would notice it, this silent implosion. He crushes only himself.