If they had spent more time cooking up the EGS ecosystem into something more similar to XBL or PSN before trying to attract consumers en masse, they likely would've been pretty successful.

That's not remotely how it would have happened.

Have a read over this article that was posted by Lars Doucet (well-respected indie developer of Defender's Quest) roughly a year before EGS even launched. It lays out exactly what a Steam competitor is going to run into trying to break into that market and provides a blueprint to not fail that is almost exactly what Epic did. And yet, the discussion to this day is still filled with nothing but "REEEEE, EXCLUSIVES!!!1", nevermind the fact that those games all still run perfectly fine on the exact same machine you launch your Steam games from (excepting, now - multiple years on from the whole kerfuffle having begun - the Deck... buying straight from Steam does make that a much nicer/smoother experience). You can even add them to Steam to get the extra features like the controller customization and such.

Basically, even if they built a launcher that was better in every conceivable way than Steam, nobody was going to switch. They had to do something else to bring both devs and players on board. As the article states:

Even if every aspect of your service is better than Steam's in every possible way, you're still up against the massive inertia of everybody already having huge libraries full of games on Steam. Their credit cards are registered on Steam, their friends all play on Steam, and most importantly, all the developers, and therefore all the games, are on Steam.

It wasn't really even exclusives technically. It was explicitly Excluding-Steam exclusives. It released everywhere else but not on Steam. And it was further aggravated by games that were already on Steam being taken off in favor of launching elsewhere.

Exactly, Steam got where it is because it managed to be more convenient than piracy (as Gaben himself said, piracy is a service problem), as did Netflix before the fragmentation (and rampant enshittification) of the streaming market made piracy once more the most convenient (and better quality) option.

Epic store exclusives don't promote Epic, they promote piracy, as that is the second most convenient option after Steam (it's worth mentioning that Steam also acts as unobtrusive DRM; infect your game with malware like Denuvo and suddenly piracy again becomes the more convenient — even the only reasonable — option, as cracked games perform better and are more stable than malware DRM infected ones; Steam provides a good enough and, more importantly, harmless option for both consumers and developers, something no alternative, including piracy, has managed to achieve).

And, of course, the instant Gaben retires and Valve goes public and begins to enshittify itself we won't be going to Epic or GOG (unless they manage to replicate what Steam has achieved), we'll be back to sailing the high seas.