And technically speaking, those computers can run calibre too, if built against against a copy of Qt5 that was compiled without SSE2 support. Okay, performance might drop.

But... old computer...
For that matter, my distro (ArchLinux) builds Qt5 without SSE2 on the 32-bit builds -- which means I suffer that selfsame "degraded performance". Haven't noticed a slowdown, really.
I believe Standard Operating Policy for linux distros is to build Qt5 that way. Of course, they also tend to patch calibre, so pick your poison.