> - Uses PHP 5 spaghetti code with a handful of garbage event listeners and output buffering
I mean it has listeners for almost every event you might want to interupt and trigger something on that are well documented and have been for decades.
The PHP 5 code is very much legacy so I will grant you this.
> - Has no integration with composer at all
Composer support is absolutely fine and there are well maintained ways of keeping plugins and themes under Composer and have been for many years.
> - Doesn't have any form of content types without plugins
Yes, but adding further content types at the level of code or these plugins is easy, trivially so.
> - Has no cache, no theming engine, no multi-language, etc etc
It has several caches that are very powerful indeed, including those included by default. PHP is a theming engine.
Internationalisation built into the core, I'll grant the multi-language support is not as good as it might be. Could be better.
> - Beyond a media library it has literally no aspects of a "content management system"
I mean, people use it to run newspapers, so not sure if this is true. It depends what you need.
The organisations behind WordPress decided to meet their user need and provide a really good, well understood platform for content management with an extensively tested UX experience of end users. They prioritised this over developer niceities.
If the purpose of software is to augment human capacity and be genuinely useful, this was the right decision. The rough edges from a developer experience perspective have been largely smoothed off by a well established ecosystem like Roots.