That’s a private gain at the network’s cost. For a challenger, their decision to bar that albeit limited form of free-riding is perfectly rational. For a dominant network, I am much more sympathetic to the call for anonymity.
Network effects make these virtually indistinguishable. If a network is unusable it doesn’t help anyone.
There may be an argument, however, for charging for the opt out.
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.