That's a very good pickup by Sony. Smart move.
After Quantic Dream and Supermassive went multiplatform, they knew they couldn't take chances with Insomniac,.
Sony's interest in QD most likely dropped due to QDs workplace scandal. Superrmassive is still making Sony exclusives, buy have in the past had a couple of non-Sony published titles. Not being able to continue the success of Until Dawn, with some poorly made games, is the indicator here that Sony may not have had interest. Still, interest may exist going forward with them.
On the first part, I think that if the intent was to bolster their portfolio, they could have done it anytime since 1998 when they started working with Sony. Insomniac has been with them for a long time. Why didn't Sony buy them after Spyro's success. Why not during the heyday of the PS2? Why not soon after Spiderman because a huge hit? If this was solely about making their first party line-up stronger, they had every opportunity to do it, even at points when Insomniac was far cheaper?
Now, on the threat of them being bought out, maybe, but you have to consider it from Management (and possibly the Board's) position. Sure, Sony doesn't have to spend the capital and still get the benefit of working with them, but then they run they risk of someone coming up and buying them away from Sony. And you may think "They'll never get bought by someone else as they've had all their success with Sony", but you have to remember that Insomniac is ultimately beholden to their shareholders. These people invested a lot into the company and expect a return, and it can be hard to get your money out of a private company as there isn't a highly traded secondary market. So if Epic or Microsoft come by and offers 5 times book for the company, then they can't say "Nah, we only want to work with Sony." They are obligated to, at a minimum, consider it. And that is ultimately what Sony is weighing here. Do they keep their cushy relationship going and hope no one else buys them up, or do they buy them up now to make sure they stay with Sony forever? Acquisitions are hot right now and it doesn't show any signs of stopping. Management of Sony (and Nintendo) are aware this is going on and they are going to respond accordingly. This is why it was important to buy up Insomniac now. Thinking it's just because they want to have the best first-party line up is more of a fan's outlook on the situation.
Sony have shown interest in the past. It's Insomniac who have never budged on wanting to be independent. Who knows what has changed now, but Insomniac were in a better financial position just before being bought by Sony than they had most likely ever been in since their inception. Sony and Insomniac have a long working relationship, are bound by 2 IPs that are pretty much part of Insomniacs success, and Sony have shown interest previously in owning Insomniac.
You aren't wrong about the the industry as it is now, but not every acquisition works the same way. The reality is that Sony must have had no interest in owning Insomniac for Insomniac, who values independence, to truly consider a deal from an company they have little (or no) working relationship with.
There is more to this than just money.
I don't know all of Sony's dealings so I'm unaware if they've ever made moves before.
One thing curious about independence is companies will always say they love their independence and then allow themselves to be bought out. Ninja Theory said something similar before being bought by Microsoft. I think what is happening there isn't that these companies will say they love their independence but, in reality, have no real love for it either way. It's more of a PR statement than what they actually believe. If a big publisher comes by and says they are going to give them more security (no longer have to hunt for publishers) and the owners get a nice payday, they aren't going to turn them down for the sake of maintaining their independence. At the very least, this isn't what these companies are doing in practice.
As you mentioned, Sony and Insomniac have worked together for a long time and made 2 successful IPs with each other. This is why I don't believe it's a simple matter of improving the first party line up. As the saying goes "Why buy the cow when you can get the milk for free." The relationship worked well for the last 20 or so years. Why change it now? The most obvious answer is what has been going on around in the industry, which is the numerous acquisitions. As you mentioned, Sony, for some reason or another, changed their minds, so there has to have been something that changed to prompt this. The most obvious answer is the buyouts that I brought up before.
And, to clarify, I do think the quality and success of Insomniac's games was a reason why they bought them up. They wouldn't have bothered to protect them (as I claim) if that wasn't the case. That is something I think we can agree on. But where I disagree with a lot of people in this thread is the idea that that's the primary reason this took place. When you say it's more than just money, I have to disagree as this was ultimately a financial transactions. It is about the money. The question is what was the driving force for Sony to spend the money. Some say it's because they are just the best, but I'm saying it's to protect their IPs and access to Insomniac. I think there is a bit we agree on in what happened but we are taking different stance on Sony and Insomniac's priorities.