Comment by kazinator

Right; that's a better framing.

1. Some people don't use tools. (For instance, we have spellcheckers, but people don't use them or ignore them.)

2. So of what use is another tool? (Why develop a grammar checker?)

See the problem? For the people who do use tools, there are useful differences among them. Some users of spell checkers will welcome the grammar checker.

Some of those who would use drugs to prevent transmission will gladly switch to the vaccine, which is more convenient for them.

But there is also this: the inconvenient drugs that block transmission are mainly used by HIV positive people and people who are exposed to them. Someone not in that situation is not going to get on a train of injections every three months. (I've never even heard of these drugs until this submission and its discussion threads.) Whereas a HIV vaccine could be something more interesting to the general population. There will likely be some people who get the vaccine who ignored the transmission-blocking drugs.

Sometimes when you add a different tool to the arsenal, non-users of the tool arsenal come around.

E.g. lots of DIY tinkerers use USB oscilloscopes, who have never owned a big one. The invention and marketing of the USB scope turned them into users. The vendors of that could have axed the idea by saying, what's the use of this idea: we already have oscilloscopes? Those who don't have them already are not going to be interested in this.

There are right tools for the right job, but also the right tools for the right people in the right circumstances.