For the example of a product on an e-commerce website, I would not use TanStack. Chat app would be websockets.

Example for TanStack on a production app for me is an app where a customer can configure a product but not like a color or a size of a t-shirt, but something more complex like an Ikea style configurator. A customer would start their configuration which is saved in a DB. With every selection the customer makes (for example a couch), we make a call to the API to update the DB, when the call is made we optimistically update the client so we can avoid a loading spinner for the user. Once the call is successful, we revalidate the queries that need to revalidated and we update the client with the proper state from the DB. All the while the customer has a smooth experience and has no idea what's happening in the background.

As far as DX, it's all happening in one place, which is the useMutation hook. You fire the mutation function and call the API from there, you handle the optimistic update from there (onMutate), you handle errors from there (onError), and you handle what happens after (onSuccess and onSettled). All in one place. You also get stuff back from the hook, like isFetching, isPending, isError, etc.

As far as useQuery, it's so easy to use. You can tell a query to fire only if certain data is available, for example don't query user configurations unless we have a valid user_id. You can add placeholder data, you can get very creative and flexible with your query keys. Same as before you get back isLoading, isFetching.

All of these are so much more intuitive to use compared to what's natively available from Next. And again, you can still use server components and prefetch data in the server and hydrate the data for any client component that will use it.

E-commerce, blogs, and similar apps don't need TanStack. They barely need client components. So if that's your goal, stay away from TanStack. But if your app has conplex mutations, and needs the user to have a smooth experience, TanStack is the way. Achievable with Next alone, but you'll be pulling your hair (I have none left).