And from Instagram's POV, the ROI on that would be much less than putting in the sort of belts and braces that the article talked about.
They don't have the time or space to engage in a massive technical debt reduction program, they're too busy destroying Snapchat and other competitors, reacting to TikTok, implementing an entirely new IGTV video service that provides their customers (ie advertisers and marketers) the equivalent of youtube within the Instagram universe, etc.
I'm sure that every large internet service's codebase out there could be made much leaner and smaller. The question is whether that is worth their while.