Perfect example of when it should have been made of a bunch of smaller fireworks displays, so that people wouldn't want to travel as far but would still be able to watch a fun show.
There are other fireworks displays in London (or were last year, anyway). Battersea Park, Alexandra Palace and Crystal Palace have/had displays. There are probably several others I'm not aware of.
The big display on the Thames is broadcast on national TV, and had 16 million viewers last year.
But firework enjoyment doesn't scale linearly. Launching a single firework at random places in London is not as impressive as a bunch of fireworks at one place in London.
Large fireworks displays do scale sub linearly, just not all the way down to a single bottle rocket. The issue is large displays are viewed on a roughly flat surface aka the earth, but irradiate light into 3D space. To maintain a proper viewing angle fireworks need to be launched ever higher into the air as your showing them to more people.
The big display on the Thames is broadcast on national TV, and had 16 million viewers last year.