Having been on the side of thinking I was absolutely irreplaceable on a team, I was more or less proven right when I left and the team failed to deliver anything at all for about 10 months, even having to shut down existing products, and is now limping along.
The truth in retrospect is that it was my fault (and my upper leadership's) that I wasn't replaceable. I created a knowledge silo around myself since I wanted to move fast and figured I could prevent the team from being bogged down in complexity if I just handled it myself and while that worked in regards to delivering out-sized results for the available bandwidth, it also was a risk that materialized as described above. So while I do believe that everyone should be replaceable and it's their responsibility to be, it's not always the case and products can live and die by it.
The truth in retrospect is that it was my fault (and my upper leadership's) that I wasn't replaceable. I created a knowledge silo around myself since I wanted to move fast and figured I could prevent the team from being bogged down in complexity if I just handled it myself and while that worked in regards to delivering out-sized results for the available bandwidth, it also was a risk that materialized as described above. So while I do believe that everyone should be replaceable and it's their responsibility to be, it's not always the case and products can live and die by it.