IMHO most of Google's woes stem from the promotion system. The incentive is to launch, promote, replace so you can get other people promoted. There's basically zero incentive to build anything durable because if you do, it means your people won't get promoted, they'll leave, your headcount will disappear, and eventually you'll find yourself marginalized and forced out.

They built a ton of stuff after those researchers were hired, including original gchat.

They stopped being able to build a chat app after different people were hired.

I think it's happening at every tech company. They are now being led by tech-illiterate company hopping businesspeople rather than passionate engineers.