Because you actually are using WhatsApp?
> Some people would pay for this convenience
Some remember when this used to be widespread and free: https://www.pidgin.im/
Beeper is just a skin over Matrix, and they are hosting and managing the bridges for you. So that when any of protocols they support breaks because the other company changes something (or intentionally breaks things to stop connections from clients they don't want), Beeper fixes it rather than you having to wait for the open source community to do so and you implement that fix. Which who knows how long either scenario will take.
Paying for Beeper is not having to manage the self hosting, which many are not willing to do. Because as with most open source solutions, it's not very simple and is intimidating for many. Not to mention having some box open to the internet, and having to maintain security updates for that box.
I would love for libpurple to come back and make these things relatively simple. But the reality is that companies have walled off their gardens to control the experience, and many other reasons ranging from greed to security to spam mitigation.
My IM situation is only currently tolerable because all of my frequent contacts are either via WhatSapp or twitter DM.
Even having the handful you do would be enough to make -me- want to consolidate.
(this is not at all meant to be an argument that -you- should want it, but I think is at least a good case that there will be people out there in your -situation- that will)