If a network is not usable by anyone with a smartphone, it's strictly less usable than WhatsApp. That means in practice, it's unusable.
Even if that was the case, " it's strictly less usable than WhatsApp" is not true, because other messengers can provide other quality of life features.
" That means in practice, it's unusable." is not true either. Remove "Status" support from WhatsApp and you get strictly less usable than WhatsApp. Still, it is not unusable.
Signal is pretty close to being strictly less usable than WhatsApp btw.