Developers lives, not necessarily users. It was also a huge waste of time and money for all parties as it achieves nothing - Facebook are the market leader with WhatsApp both globally an in Europe - iMessage barely registers in the top 5.
The law isn't specific to Apple, it also affects WhatsApp. Indeed in the EU that's probably much more important. Personally I don't know anybody who consciously uses iMessage, but the extreme network effects of WhatsApp make it socially impossible to move away from.
Why wouldn't it improve? Currently none of these services offer interop; in the future they might. That's a static upgrade to me, and I'd imagine most other people too.