People only leave a workplace for another if pay, benefits and personal enjoyment is better elsewhere.
When you start a new studio like the Initiative, it's unproven. So you are making risky career moves by moving from an established studio to go somewhere else. Which makes me think that their previous places of work wasn't that great when it came to pay, benefits, hours, management and/or fulfillment.
As a worker, you are not married to the company. Just like they can let you go, you have every right to leave if someone else wants to give you a better deal. There is nothing scummy about that.
Seattle is a good example. When Amazon Game Studios started mass hiring, they got top talent from all over the city. That's great. Amazon is burning money, one of their games have already been cancelled, less senior people are moving up at the previous companies. It's great.
Games are a massive collaborate effort. You can snatch talent from other places, but that in itself doesn't mean anything. Over the years we have had many "dream teams" that didn't produce anything of value. Look at 343 Industries - It was a collection of some of the best developers in the world. Didn't mean they were able to make a Halo that topped the OG games. That shit doesn't matter.
Making games takes hundreds of peoples, and there is a lot of things to go into making games besides the talent of the people. Just think about logistics, sound preproduction like picking the right engine and tools before you even start production. That lead design engineer who decided on the workflow for how they were gonna deliver content for a project 3-4 years down the line. That correct decision right there is probably worth more than a gazillion top talented designers.
With Destiny, Bungie had activision throw endless amounts of money after them to get the best people to do whatever it took to wipe the world with Destiny. Who gives a shit? Talented devs cannot do shit with spaghetti code. Everything takes forever. You go nowhere but at snails pace. Someone fucked up before it even began and the rest of the games life will suffer for it. It happened to the Old Republic too. Bioware Austin was a star-team of top MMO Devs and star developers from top studios from all over. Couldn't do shit when they were hammered by that terrible HeroEngine. That was a garbage scam, and it fucked up what could have been an incredible game.
Also, you think about what it is like working in software development. Most developers will never work on anything of significance. Most developers will never ship a game that anybody has heard about. Most devs work on bottom-tier iOS games that ADHD kids are mass downloading on mommys apple credit card when they are not watching. Most developers would saw off their leg to have a credit on a game like God of War.
So you have to ask yourself what would make those devs at Santa Monica, decide to jump ship to a unknown upstart. Working in AAA can be a bitch. A never-ending crunch of fuck. Its not uncommon for workers lives to fall to pieces- Particularly their marriages and relationships because they are never home. And on these games you are expected to be in crunch for years. I dont know if it was like that on GoW, but I wouldnt be surprised if it was. There is a reason why Cliffy B, Jaffe and many other top designers jumped out of that even though they could have kept staying in the spotlight making the top games. Because it fucking sucks the life out on most people. And that is even more so for non-leads who don't even really get the recognition.
So if a competing studio offer you better hours, benefits and pay. And more vacation. Shit man. Take that shit. If Amazon or Microsoft or Apple have money to burn and you will be treated nicer? Thats not being scummy. Thats putting value on workers health and well being. And we need that in software development regardless of where you are. Everyone deserves to be treated better. QA, Design, Support, Programming, whatever. These major teams can sometimes shove people together like cattle, and just work people into a pulp. Unreasonable budgets, poor management, bad deadlines, stupid preliminary decisions that fuck up the project long term. These are the things that make life hard for people on the ground.
If Sony values their worker (or any other studio) they will pay them better. They will give them better hours. They will make sure they are less stressed. They can start by hiring the appropriate amount of people and stop "expecting" people to work an absurd amount of hours. Particularly because the productivity of workers goes to shit after a while. So it doesn't even make sense.