It is, but it's not exactly a good subscription service for the average Discord user. In the past, they kept adding and removing features, trying to find stuff that stuck. While I think they've got a decent feature set right now, it's still not great, and I don't think the average Discord user would be very impressed by it.
"Pay $10/ month for increased upload size, custom emojis shared everywhere (and not tied to being used in only one server), and animated profile pics/ banners/ backgrounds!" There are other features, but I don't think most people would use them.
The biggest feature, custom emojis being able to be used in any server, is also part of their $5/ month tier, so most subscribers don't even have a need for the more expensive tier.
To a degree, yes. But a non-public company doesn't usually have that "obligation" for ever-growing profit. Unfortunately, Discord's goal does seem to be to eventually get an IPO.