I'm not gonna rehash the full deal, everyone knows the "PHP is a fractal of bad design" article. Whether you think that's an exaggeration of the problem's impact or not, it's hard not to agree that PHP has a lot of features that are counterintuitive, encourage bad coding practices, and make modularity/expansibility of the codebase a real challenge. PHP is fine if you do it right, but it's really a challenge to ensure that a bunch of volunteers are doing it right, 100% of the time. It's not a coincidence that the people who are using mega-scale PHP codebases like Facebook aren't actually running PHP - they've gone through and cleaned up some of the awfulness and run a language based on PHP. Those are the design decisions that the actual PHP standards group isn't competent to make, because anyone who's sane has run for the hills.
Discourse is indeed extremely overengineered, but I don't think the urge to start over on a modern language that has better design features is wrong per se. If you had a web forum written on ANSI C or COBOL, wouldn't you say that at some point the costs of an inappropriate platform or a limited pool of engineering talent outweigh the cost of just rewriting it? You don't have Facebook-level resources to refactor the whole platform, you don't have billions of dollars of sales riding on your software. It's a comment forum. To throw your comment back - if it's just a simple app to display text on a page, why not fix the technical debt and put yourself on the right long-term track?