Sorry; I disagree. Since moving to digital, my first tele was a kit 55-300mm to which I would agree with your conclusion - that larger birds and animals were possible; smaller ones: not so much.
However the 150-600mm APS-C, (i.e. 225-900mm FF) has been quite capable for me. Witness the signet here or the fairy wren here - taken at over 11m, a very small bird.
These are prerequisites for wildlife photography and for birds in particular.
Sometimes, as mentioned elsewhere in this thread is that 150mm is not wide enough - when I went whale-watching 3 years ago I captured some bucket-list images; but I also found that I could not always fit my subject in the frame!
I believe it is always possible to lust after a longer lens, or a faster (optically) one. If someone is making their living photographing wildlife, then any price, size, or weight can be justified. For this hobbyist on a limited budget who likes to take wildlife images, 150-600mm (on APS-C) has been for me the sweet spot. That does not stop me dreaming of something like f/2 or thereabouts at the tele end… GAS never ends.