> They have thousands of engineers that in the span of a decade don't seem to produce much at all, nothing visible or memorable anyway. Long-lasting Twitter problems (culture, spam, algorithm issues) never seem addressed.

Why do you assume it's engineering problem? I think engineers at twitter are well capable to solve any problems, but it's just not a business need. The real problem are product owners/stakeholders/business people, incapable to transform, envision and lead at Twitter.