That's definitely a very good question. So I guess the first thing I will say, and this will probably sound a little bit pedantic, but Nostr is a protocol, not a platform. And that is a very important distinction. platforms are usually centralized entities. Nostr is decentralized and infrastructure is mostly run by people in the community who care about it. 

As for the first part of your question, how would it support a high-inflow of users? Honestly, I don't know. Kind of circling back to the decentralized nature of Nostr. Considering that pretty much all of the infrastructure is ran by enthusiasts, It genuinely depends on the strength of their servers. Smaller relays ran at home on a Raspberry Pi, or sufficiently weak hardware might have problems if a shitload of users decided to connect to them all of a sudden, some of the larger relays ran on cloud infrastructure, could easily scale up and probably deal with a high influx of traffic. 

As for the other part of your question, which kind of sounds like the problem of onboarding new users. I think the only proper answer that I have for that is it's still a work in progress. Onboarding new users is something a lot of client developers are actively working on. I think partially it will come down to education considering the fact that a "nostr account" is actually just a key pair and if you're only used to the username and password model of doing things explaining a key pair can be confusing. So I don't really think advertisement in the traditional sense of advertising is probably the best way to promote nostr. 

Also as you probably realize content and user discovery is weak at best. And still needs a lot of improvement. Hashtags seem to be the most useful at the moment But again, discovery is a problem because discovering the hashtag that people are using for whatever type of content you want is not easy at the moment. 

The client interoperability issues partially stem from the fact that Nostr is a young protocol and most of the protocol is not set in stone yet. They're still trying to figure out ways of doing things And people being people, everyone has their own opinions on how things should work. 😅

So I guess, in short, I'm not exactly sure that nostr would be ready for "prime time" of millions of users coming on all of a sudden. But at the same time, having a high influx of users would help battle test the protocol and figure out how things can be done differently if we had more people using it and giving developers feedback on what works and what doesn't. 

I hope that helps. I'm personally excited about Nostrs potential, but I also realize that it's current limitations And non-standard way of doing things Will be a hurdle to overcome.  

 Wow...I really appreciate you taking so much from your time to write this. 
I understood your answer, and I think you are right about the risk I mentioned of having that high inflow all of a sudden, not being something to take into consideration at the moment. And as you said, I also consider that the best of knowledge on how to improve things will come through time, surpassing different types of tests/difficulties. 
What I hope, is that from this end, the user end, we could actually share ideas to its creators on how to develop this, to make it better (imagine feedback actually taken into consideration somehow, maybe through some filters, I couldn't think right now of a specific frame of how that would work, but it could be doable). 
To me, this protocol, in its main idea, which I think it was built on, this is something I wished for a long time ago, and I hope it will actually be that thing. If not, maybe morph into something better. Nevertheless, it is something really good, and I hope it will keep growing.