Scaling, reliability, support, dialects and localisation, accessibility, edge cases, device support, security, moderation (not just moderators but moderation tools) infrastructure, research and exploration, monetization, commercialization. Basically everything no one in a startup thinks about for years if not more. And to that add the simple fact that as the team grows the per engineer productivity just has to drop due to communication overhead among other things.
Yes, in the early days it ran on 6-7 different platforms. Back then it was about 1-3 developers per platform, however. Small focused teams like that can have significantly higher per-developer productivity, but are far more limited in the breath and nuance of what they can work on.
As a project grows in scale, there are a lot of development tasks that take a significant amount of time and effort for relatively little user-visible impact. However, in aggregate, there is a benefit to investing in those sorts of things.
For me WhatsApp has gone from my favorite app to something I won't touch except almost as last resort.
I had no issues with old WhatsApp whatsoever and I'm not aware of anyone else having any either and I'm part of multiple groups that used to use it extensively.
The only thing that has improved meaningfully since then is end-to-end encryption, but as much as I love that it is only nice-to-have for me and I would feel a lot safer with just ordinary encryption as long as Facebook wasn't snooping in my contacts and metadata.
