Steam

You're of course right with the exclusivity argument — that's a very real possibility, and yet Microsoft has tried it with Call of Duty, one of the most popular franchises ever, and saw very little success with it, resulting in them putting it back on Steam years later. If I were to guess why attempts like this have failed in the past, I would say that Steam is so dominant over the PC gaming market today that not even large franchises going exclusive attract enough of a user base to offset the loss of customers that aren't buying games only because they're not on Steam. Add to this the additional overhead of developing and maintaining a competing store front, and the cost-benefit analysis leans clearly towards just being on Steam and accepting their cut of sales. The exclusivity tactic clearly failed even for big titles like CoD, so it definitely won't work for smaller ones. And we're not even talking about cutting into the indie game market, which would require making very attractive exclusivity offers to many smaller studios, all for acquiring exclusivity on titles in the hope that they'll be the next big hit — a very high risk strategy that likely results in a lot of sunken cost short-term.

Once they have that market share, they can give developers better margins, since they'll be selling customer data at a profit

When we talk about "selling customer data", I think we need to look in more detail into what this would actually mean in practice. It's very unlikely that any online storefront could legally literally "sell your personal data" like address etc. that you would enter presumably as part of the payment process to third parties. That's just illegal almost everywhere in the world, and certainly in the largest PC gaming markets. It wouldn't lead to significant revenue either, because raw data like that just isn't very valuable. Instead, I suppose what people mean when they say this (in the context of companies like Google or Facebook) is just the practice of selling advertising services that use the data they have on people to advertisers, who can then target their ads at highly specific segments, improving their return on ad spend. The actual private data though stays with the entity that collected it — because it's what actually gives them the edge on the market; it allows them to offer better ad targeting than competitors.

How would this apply to Steam or a potential competing storefront? Barely. I assume no-one is arguing that a steam competitor could launch a generic advertising network that could stand against Google or Facebook, so we're probably talking about advertising within the storefront itself. Steam today already collects information on your interests and customizes the store based on that, plus presumably your location, age group etc. — so they're pretty much already using your "personal information" to the extent possible in this context. How else could a competitor realistically monetize personal information?

It's a market, markets trend towards short term gains strategies over long term gains strategies because having faster short term gains means you can more easily crush your competition.

I wouldn't say that this is the case when we're talking about trying to eat into the market share of a dominant entity like Steam. Sure, potential competitors can make short-term plays that cut away some market share, but such strategies are expensive, risky, and alone likely don't lead towards a significantly improved position long-term (exhibit A, again: COD being exclusive to Battle.net).

For better or worse (usually worse), toppling a near-monopoly like Steam is extremely hard for players with big cash, and practically impossible for independent competitors. This is especially true for products that are inherently sticky, like Steam, where people have curated large libraries over decades. The only reason Steam's dominant position is not hurting the consumer is because their product works well and is in many ways very pro-consumer.