For developers, they take a much smaller fee than Steam or GOG, and for players they’re constantly giving away free games.

"Free stuff, pl0x" isn't an indicator of supporting the industry or players. That's a business tactic for clawing market share away from their competitors by attracting people without the means to buy games and devs desperate for funding. Also, if parity is your worry, many games on Steam go free or effectively free (<1 USD) all the time.

That sounds like a decent reason to campaign against them. I haven’t heard anything about that before. What was the story behind that? (As in, when / why / how / what? Perhaps you have a link or something.)

With Reddit going tits up and a coverup operation by Epic throwing a bunch of garbage info around, it's been difficult to find the exact sources (why I've been taking so long to reply). If I find the actual articles/posts I'll link them, but in summary:

• EGS bypassed many APIs, such as Steam's API, to data mine your usage statistics of their competitors, including friends and games played - they didn't ask for your consent nor Steam's.
• Some major red flags with memory manipulation and internet traffic obfuscation.
• They "apologized" about it, citing some bullshit reasons for that behavior. Suspiciously, behavior changed.

I’ve heard people talk a lot about exclusivity contracts… but can you name even a single game that has such a contract? [...] Like, is there actually something bad happening here? Or are people just speculating that something bad might one-day happen if Epic got bigger?

There are loads of games in my "do not buy unless heavily discounted" list precisely for taking exclusivity deals. Hitman 3, Darkest Dungeon 2, Hades, Satisfactory, among others. The danger, beyond rewarding shitty behavior, shutting out large portions of the community, and limiting consumers' options, is the same as always - you're effectively telling companies that whoever has the biggest pocket gets to dictate what the entire industry has to do.

But Steam has been around for some 20 years. It’s hard to catch up with that so quickly. In any case, although missing features is a good reason to prefer Steam, it certainly isn’t a reason to campaign against Epic.

It wouldn't be if Epic had shown any intention of eventually having parity. It's been however many years since they released, with the immense advantage of seeing what works for Steam so they could copy it, and yet their client remains just as bad. It clearly shows that their focus in on getting market share to exploit gamers and devs, not on making the best platform possible.