> By volunteers adding more instances(that they can close anytime)?
"That they can close anytime", just like twitter in this example. Partly, yes. But that sort of total exodus would mean a lot of additional people contributing ideas and code to the Fediverse, not just servers, but by making it easier to run your own instance. Who's to say it couldn't be run just like an email client with the right ideas and effort? It's such an extreme example that I'm not even sure it's useful to discuss.
What traffic is it prepared for? It would be interesting for you to provide the numbers and the evidence which backs this up.
As for real life scenarios...there are upwards of a million people using it right now. I've made friends, networked professionally and found several homes there. I am literally a real life scenario and so are the people behind most of the posts there.
And today you are right about "anyone can start their own instance", but it's a darn sight easier than running your own twitter.com, and it'll get easier every year.