I am no expert on Discord, I frankly have used it little. But my impression is, it's a replacement for subreddits? Not a replacement for reddit in the whole? Is there a single feed of everything that's popular within Discord? Like the equivalent of the reddit front page?

No. Then again, I never use the reddit front page, and don't care (personally).

If subreddits move, reddit's front page won't avail them much.

So one would have to check every channels on every server they subscribe to, every day, just to see what's new? And you consider that a replacement for the Reddit home page?

I have several hundred subreddits, that would take me forever on discord.

While it’s not a 1:1 replacement, I have to agree this is the most likely alternative for niche communities to live. It’s already common for certain subreddits to have a discord server and direct users there.