iMessage is a vestigial product from the days when people had unlimited data but also only 400 text messages a month. It made sense then. It doesn't make much sense now.
It still makes sense over SMS/MMS which are terrible protocols in comparison. Even if you get unlimited messaging now, group and media messages are still remarkably unreliable.
iMessage is not a protocol, it's an app. Nobody uses SMS because everybody uses a messaging app. It comes down to whether you choose to use a cross-platform one or an iOS-exclusive one.
I think it is less that we have more SMS allotment and more that nobody wants to actually use them anymore, because they're insecure / don't work on wifi / are slow / etc.