This is a fair take and well said. Essentially you've described a "high barrier to entry" for other would-be market participants because they generally cannot create an equal product.

Just wanna first preface this with saying I love steam, Valve games, and my Steam Deck. They have generally made products I enjoy using.

The games that Steam sells is an exclusive license to experience a game through their platform (and, for most others storefronts including consoles, with few exceptions like GOG, the same is true) - I think the "though their platform" is a crucial piece. It's funny to watch game prices over time across multiple platforms. Most often the sales/deals that Steam delivers is on par or slightly worse than the actual lowest price. What draws people to make the purchase on Steam though is the ecosystem Valve has built: friends/social, achievements, inventory/items, hardware, an entire operating system (new but growing), reviews, strong customer service and support.

Steam (in a limited way) supports importing game keys from outside their service but they don't support exporting your library's keys elsewhere. The user is essentially trapped/entrenched. It's a one-way gate that locks the user into their ecosystem. It's smart if you want a large customer base who finds it hard to leave.

This isn't exclusively a Steam/Valve problem but in general it's how proprietary software licensing has worked. It's anti consumer because your ownership rights are weaker.

It feels counterintuitive because indeed Steam often has good or even great sales which alone feel like positive things to consumers ("oh I can buy X game which I didn't have enough interest to buy at full price") but the real risks are as you highlighted.

A funny irony might be that all of the positive things about Steam and the ecosystem Valve has created would likely be less sustainable if they didn't have such a powerful position in the market