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.
I think this is a bit of a strange take on the situation. Firstly, the entire MO for Sony is to work with a studio before buying them. Saying Sony would not buy Insomniac only to bolster their portfolio is just a complete misreading of how Sony usually acquires studios compared to how MS is doing so now.
Also, there was no way Insomniac was ever in threat of being bought off by MS that Sony would have to defend against losing them. Sony is pretty much the only publisher Insomniac has seen actual long term success with, and don't forget that Sony were the ones who offered a Marvel IP to them ad they chose Spider-man. They were pretty much gifted it due to their working relationship with Sony. There was no way they were going to lose access to SM or Ratchet just to go with MS.
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.