Let me rephrase; I'm pro-Xbox.
Obviously as a person who primarily plays on Xbox, there are benefits I'll have. But, looking at gaming in general, I see more posivites than negatives. And this is me
trying to be objective. I can't be totally objective, no matter how hard I try, but I'll try...
1 - It increases competitiveness. Sony being one of the market leaders will be forced to bolster its own offerings and services, and perhaps improve where it is weakest, i.e. its subscription services. That's great for PS customers, which I also happen to be. If these improvements show great yields for Sony, it might push Nintendo (and others?) to strengthen their weak areas as well (subscriptions). Now, I'm not for or against the subscription models, unless the option of purchasing a game outright, is completely removed.
2 - The purchase, while it does wonders for MS, doesn't automatically push them in any lead, whatsoever. They'd still have a lot of work to do, and while it might set precedent for them to look at more acquisitions; it won't do so without concession. This was a big play for them, but because they were weak in the console space, is the only reason why this is going through. If they were market leader, this deal might be have been blocked, with valid reason. So, the status quo isn't broken, but it puts MS in a better position to compete, which is good for consumers, or at the very least, MS customers.
3 - It brings Activision titles to more platforms.
There are some potential cons and some outright cons. The potential cons aren't guarantee to manifest, but they're valid. MS using their AKB catalogue to help monopolise their place. It'll be hard though, as they can't simply wipe out the competition with this purchase.
Secondly, while PS only has COD for 10 years, and there's a real chance that MS might make it exclusive after 10 years; that's still a 10-years-away supposition. The gaming industry, while it has its formulaic matters of predictability, it's an ever-changing landscape. There's no telling if COD will retain its strength in 10-years. There's no telling if PS (or some other company) will launch a new FPS that completely turns things upside down. Or, at the very least, bring some more competition to the FPS space.
The last potential problem is MS going on an aqusition spree. I think they have to be extremely strategic about this. Their strength in cloud can work against them, just as PS strength in console worked against PS in this deal. However, they may only have 1 other big acquisition (with concessions) before any other big merger is outrightly blocked. Their numbers have to show that any big merger won't put them in a position to monopolise their power, and clearly this one doesn't. But, moving forward, anything else will likely be shut down instantly, unless it's perhaps a Japanese publisher.
I'm agnostic to a company's means of success (the old "home grown Vs buy-outs"); either method is fine. Use the tools you have at your disposal.