I’ve worked with Firebase before and really enjoyed it (disclaimer: I’ve only done a little bit with it in React Native- most of my experience with it is in Swift). It’s my first choice for building something completely from scratch. Sync is always complicated, so having a platform that abstracts offline/ online syncing is a huge boon to productivity. Like anything, you can setup stuff in a way that doesn’t scale well, but I have no doubt Firebase can scale to 100k or much further. The free tier is very generous and gives you tons of runway to try something out. If you’re building something on your own or with just a couple of people, IMO it’s really important to pick a few things to focus on where you want to put your most effort in, and simplify/ outsource the rest. With Firebase, you basically eliminate the whole step of having to write a backend API so you can focus on the frontend user experience. That’s huge.
In terms of cost, whatever you choose, I wouldn’t worry too much about what 100k users would cost. Or even if your app could technically scale to 100k users. I would worry first about building something that gets 100 devoted users (which the Firebase free tier could handle easily). If your app gets popular beyond what you can handle, then hopefully the idea has legs and could be monetized/ turned into a full-time paying job/ get funding. And then there could be resources to improve the technology to make it scale better. Most apps never get to this stage.
I’ve found Pieter Level’s philosophy on a dead-simple infrastructure for new apps interesting. In this blog post, he talks about storing app data on his server in JSON files- no database. In other places, as I recall he’s mentioned that you can actually scale surprisingly far with just a SQLite database.