"The main way that shpool differs from tmux is that tmux is a terminal multiplexer which necessarily means that it offers session persistence features, while shpool only aims to be a session persistence tool. In contrast to tmux the philosophy of shpool is that managing different terminals is the job of your display or window manager, not your session persistence tool."
What if the computer user does not use a window manager. Many years ago I stopped using X11 and started using textmode only. Then tmux was released and appeared in NetBSD packages collection, it soon went into NetBSD base and I have been using it with almost zero problems ever since. If I am forced to run a graphics layer so that I can run a window manager so that I can run shpool, then is this solution truly "lightweight". The shpool program might be lightweight but the system I would need to run it would be heavyweight, when compared with textmode, no graphics.
For me, an assessment of "lightweight" also needs to consider the compile-time requirements. For example, I use a static copy of GCC that weighs in at 242.3 MiB and I can obtain libevent and libtinfo, the only two third-party tmux dependencies, from any number of mirrors across the internet. I like that GCC does not try to connect to the internet by default, or otherwise. When I lose an internet connection, I use the time to compile programs. GCC, and probably most if not all C compilers, add no friction to doing that.