Interesting development. We learned a few things. First of all, Tim Sweeney and Epic are no longer neutral in the competition between the console manufacturers for market dominance. By accepting this deal, they have tied their financial well-being to Sony's. It's now in their direct interest for the PS5 to be as successful as possible so their new shareholder will be content and not interfere with any other goals they have and it's in Sony's interest for Fortnite and the UE tech to be as widely adopted as possible. None of that is illegal or dishonest except the part where Tim Sweeney was calling the PS5 revolutionary and comparing it to high end PCs at the same time that he was negotiating to raise as much capital from them as possible. I don't know what that did to the share price of each company but it vaguely sounds to me like something that the Consumer protection agency and even the SEC may want to review.
It also raises the question of where that leaves Microsoft which was probably already a bigger client of Epic than Sony before all this (as far game engine tech is concerned). If I'm Phil Spencer, beyond the raw economics of it which I have analysed above, I can no longer view Epic and Tim Sweeney as a trustworthy partner or supplier. The fact that XGS has enormous reliance on the Unreal Engine is a weakness for Microsoft as of today, no matter how nice Hellblade or the next Obsidian RPG look. You can no longer blindly assume that you are getting the best that the engine can offer for your money and also you are partly making your competitor more successful just by running your business. This is something that you do when you don't have a choice (MS using Blu ray, Sony using Azure) but not when you do. It's exactly why Sony did everything they could to have Amazon as their cloud infrastructure partner before turning to Microsoft.
The logical effect of this is XGS should start exploring and if possible adopting alternatives to UE. Some of the disadvantages of those alternatives now are offset by the fact that they are reducing Xbox's reliance on Epic and this warrants a complete reconsideration. If that Slipspace engine has really costed as much as it is rumored to develop, may be it is up to the task and you start spreading its adoption in order to diversify ASAP. Just my 2 cents.
Sony is a minority stakeholder. We're talking 1-3% (likely 1.5%). Microsoft could become a stakeholder tomorrow if they wanted to. Sony (not PlayStation) owning 1-3% of Epic games isn't going to have an impact on Epic Games. Epic Games will act as it's own company who happens to be partly owned by Sony. They don't need to make Sony happy especially if it negatively affects their overall business as a separate entity by shafting Microsoft.
I think the conspiracy that this investment is related to the praise is absurd. A lot of people are praising PlayStation. Technical people have even sided with Sweeney such as John Carmack and all the people who were giving Linus shit on Twitter. Sony (again not PlayStation) has been showing interest in investing in other companies with the recent bids for Leyou. There was a report last month that Epic Games was trying to raise funding from a bunch of investors and Sony wasn't one of them. People are just upset that Epic didn't praise the Xbox at all and because of all the sensationalized click-bait articles that have headlines quoting/referencing excerpts from Interviews; and act like Epic is going out of their way to spread pro PS5 messaging.
Sony is likely not going to be a whole lot more successful because of this investment. These investment are so that you make more money than your money sitting in the bank. Sony is likely not going to make a whole lot of money from owning Epic Games apart from when they sell it. The fact that they keep raising funding, shows that they are likely reinvesting all their own profits back into the business to grow it. Not using your competitors products is an old mindset that's becoming less prevalent nowadays. Sony probably uses all kinds of Microsoft products and Microsoft uses all kinds of Apple products. Apple uses Windows PCs sometimes and used Bing and Azure for a long time and is now using GCP. Also PlayStation didn't use Azure at first because Azure wasn't really a thing and wasn't nearly as good as AWS when it launched. Worrying about a bit of your money flowing to your competitors is such a trivial thing. It'd be like PlayStation not letting Microsoft release Minecraft on their console because they don't want Microsoft to make money. As a business, if it's a better mousetrap they are going to use it.
The idea that they should look into not using Unreal Engine because Sony owns 1-3% is absurd. I think that Microsoft should develop their own engines but not because Sony owns a small fraction of Epic games. Slipspace is probably not a multi-purpose game engine; considering it's developed by the team whose only going to be developing Halo games.
Companies by stakes in other companies all the time, it doesn't mean much unless it's a big chunk. Microsoft owning both Apple and Facebook doesn't have much of a impact on them and they could care less.