I wonder how that can be, though. Either you are constantly surrounded by junior people, or...? I am sorry, but I can't believe that out of 10-15 people there is not a 20% of people who can do part of your job.
I can tell you, out of 10-15 people, there are none that can do my job.
This is obviously untrue, they can definitely do it, and there are people I’d be happy to see in those roles, but the quality of work would suffer in the short term, and we have a ton of deadlines/projects to complete.
The problem is that at some point you'll become the bottleneck and projects will be delayed as a result. Or you'll get sick, a loved on gets sick, etc, etc. Either way productivity will suffer and things generally start to spiral downwards then (people get bored, leave the company, putting more work on you, etc.).
In many places there is always something urgent and there will never be a good time to slow down. Then you end up in my first point eventually. So part of your job is to create that breathing room, push back on projects, get people trained, etc. Think long term and not just moment to moment.
edit: And if you think training people now is painful, doing so when you're actually past the breaking point will be be a lot harder and cause a lot more disruption.
I spent a lot of time training people in my previous job only for them to come back to me with basic questions about what I taught them over a year later.
What was missing was technical leadership from above: the managers didn't realize (or value) the need for the team to get familiar with the systems that they were working on, so they didn't make sure that the team learned these things well, or brought in a few more people who could guide them in addition to just me.
In this case he is the manager so he can do all those things you mentioned to properly train his team. It takes effort and sacrifice but that's his job.
