Discussions like these are highly opinion based. Opinions which do not immediately affect the product Flarum.
As such I agree with Jordan's decision to move this to off topic.
The subject is really interesting and I am a strong believer of keeping things simple. I always provide the example of a countryman called Pieter Levels who wrote and maintains two flagship products (nomadlist.com $ 25k+ per month and remoteok.io $ 20k+ per month) as a single php file.
Even so, apps that other people collaborate on or are meant for larger teams/organisations benefit from clear code separation and well defined responsibilities. Object Oriented code (including namespaces) entirely improve that situation and the daily happiness of coders involved in such projects.
TLDR, the larger your app (in terms of feature) the more reason to bring structure to chaos.