Well, for me with a lot of smaller than 1000 - 5000 concurrent users (we don’t care how many there is per day) that wasn’t enough. That’s why I needed upgrade to 4GB/2vcpu droplet — and party because I needed bigger disk too.
Now I have too many cores and only a bit over 50 % of RAM is in use 
Sure. I would use AWS or another solution for files. I don’t have global audience and actually that solution costs same or a bit more than increasing droplet at that level. And if/when one needs more RAM upgrading cost same with or without harddive space — it is only matter of ability to roll back (and if a forum gets smaller in the time passing and there is need to roll back… that’s not wanted situaton 
)
Question how much is much is almost as hard to answer than how much is enough.