Your question rephrased: "Can EMAIL handle 100 million daily users?".
The answer is yes.
NOSTR is similar to emails. They depend on nostr/email providers and aren't depending on any single of them, what exists is a common agreement (protocol). The overwhelming majority of those providers are free and you can also run your own from the cellphone.
Some providers might become commercial like gmail, still many others will still provide access for free. Email is doing just fine nowadays, NOSTR will do fine as well.
This is all necessarily true of any "protocol". It is absolutely not true that every protocol scales efficiently to 100 million active users all interacting though, so it is basically a meaningless claim.
E.g. ActivityPub has exactly the same claims, and it's currently handling several million, essentially all interactable. Some parts are working fine, and some parts are DDoSing every link shared on any normally-connected instance.
Email has some social scaling problems, but the technical scaling side is almost perfect - you push to the destination. Adding more destinations doesn't increase cost, and it splits the load.
Contrast this with e.g. Secure Scuttlebutt (under a normal setup). Adding more people to the connected graph (implying also a more densely connected graph) means exponentially more traffic and more storage for each member, due to the friend-of-a-friend proxying built in. It doesn't take long at all to reach the point where a normal internet connection can't keep up, and you fall farther and farther behind.
Mastodon falls somewhere in between - adding servers mostly splits load when you add instances, but caching behavior grows linearly and tends to happen simultaneously ~everywhere, and it doesn't take long before it's beyond what hobbyists can handle. It even affects outsiders, due to preloading link previews, which is a core privacy decision that's arguably part of the protocol.
Where does nostr fit in? Because "a protocol" describes ^ all of those. It's a mostly meaningless descriptor, beyond "probably not tied to a single company" (but not more than "probably").
Now that is unfair. You are pointing out flaws at Mastodon and Scuttlebutt and then applying the same tag to NOSTR without being correct.
NOSTR is the same as Email on that regard. You push a note to the destination (relay) and adding more destinations doesn't increase the load, it splits the load.
There is no centralization at NOSTR, there is no mandatory record of who sends who and what. It is just like email, send it over and let the receiving parties do whatever they want with it.
In fact, it goes beyond email to the point that you can write a NOSTR note on a piece of paper and still be certain that the note was written by a specific person and is unmodified.
