Why? As a web app developer or user, the browser engine is low on my list of priorities.

Browser engine preferences are about far more niche concerns.

As a web app developer your perspective actually makes you desire a monopoly so you only have to test once. But, as a person I don't want that situation at all.

I don’t know a single non-technical end user that even knows what web “engine” powers their browser, let alone cares. This is not something real people remotely think about.

That doesn't mean it doesn't matter or doesn't affect them.

Just because they don’t think about it doesn’t mean it is good for them. (E.g. sugar in almost everything that kids eat).

I didn't say real people...