I'd be surprised if many of them didn't just come back. It's kind of hard to give up talking to your friends or being part of some group who communicates in a whatsapp shared thread
Last week servers were unstable for about 24hrs. But they did grow by about 10x in approximately a week's time, so that's not entirely surprising. I think it's more surprising that they've been so stable so far.
It was very entertaining seeing all my group chats move one by one to Signal now that there is a critical mass of users that agree that it is the right choice. I tried to make it happen in the past and it was too much a pain to convince people that Signal was way better than WhatsApp.