Really? The only exception I have ever had to make to using deb sources for every kind of software I have ever needed is Bottles.

What other software is only flatpak available?

Anyway, I think flatpaks are just reinventing .exes. Yep lets do the equivalent of installing DirectX 35 times all over again. For older hardware and those with less access to internet, flatpaks are not great.

Further, because flatpaks do not have their dependencies updated as frequently, this paradigm is less secure than having all your dependencies updated whenever you run apt update.