The main problem is that back then, messaging services were just that - messaging services designed to keep people in touch. They were open, or at least neutral

The main problem is that back then, messaging services were just that - messaging services designed to keep people in touch. They were open, or at least neutral to third-parties interoperating with them. The problem is that doesn't pay the bills, or at least not as much as "growth and engagement" and stalking/non-consensual data collection does. So they started restricting interoperability with technical and legal means (somehow copyright is often used to prevent third-party clients for displaying content despite the user having a "license" to access said content and despite the original rights-holder - the sender of said content - having no problems with this). There's no technical reason why a unified messaging client can't be done, neither now nor a decade ago and in fact it was a mostly solved problem a decade ago before the "growth and engagement" crowd came around.