As a Lead:
When my wife had heart problems, I instantly told my team to go on auto-pilot, who would run their ceremonies, and went to support my wife. When I got called by someone with a question that was answerable by my team, I chewed them out. This is all with 100% support from my director.
As a developer:
I'm working on a product where the product has been developed by one dev largely for many years. I'm at the 6 month mark now, and honestly, if I had to step up to support everything myself, it'd suck, but the company would keep going. Would I have to change how a few things work? Probably. But would we get our releases out: Absolutely. Usually at 6 months, I could take over most products but this one is very specialized. (Which is why I joined up. I knew the ramp was longer, and I had more growth. :) )
And that is the real story: If you want to get that "truck number" up. Get someone else into your codebase, and working. If not expect some lag while they come on-line, but that the world will keep spinning.
... Also look for people who can learn, over just raw skills in a replacement scenario. Clearly they need to know enough, but it may take you longer to find someone who can check every box, than get someone and have them ramp up to "good enough" quickly, missing a piece or two. Heck you might want 2-3 developers.. something about learning your lesson.