Type inference can make the code easier to write. But, is the code easier to read, and overall less effort for the business? Code tends to be read significantly more than it's written. This is a simplification but seems to be true enough; modification by subsequent maintainers involves a lot of reading.

I worry about features like overuse of type inference when it means the types in question aren't explicit from the code that you're reading (agreed, not all type inference involves overuse). For example, if I write:

  var x = f();

What's the type of x? What's the return value of f? I can answer those questions, certainly, but I have to go look for the answer. I don't immediately have any mental concept or name to hang the idea off of. Whereas in a language like Java, I'd end up writing:

  Foo x = f();

Now I know the return value is a "Foo". This may not seem like much, but suddenly if there are many different Foos in the code, I can see the patterns I didn't see before, and I can draw correlations:

  Foo x = f();
  Foo y = g();
  h(x,y);

Everything can always be understood with a degree of thought, but type annotations seem to, in my experience, make programs a lot easier to read and modify. The original authors typically have the whole thing in their head, so it doesn't make a difference to them, but it can be a huge difference for those who come after.

There's a certain simplicity that comes from types that are so concrete, clearly named, and linkable as Java libraries are. You can figure out something about this code without reading the declaration or definition:

  ListenableFuture<String> emailAddress = user.queryEmailAddress();

You have a pointer for what to search for and an immediate name for the concept. I suppose it's like an index, in a way. An index to make code more readable and referenceable is included throughout. It's hard to explain it, but I find this valuable even when I have support from a sophisticated IDE. (Plus I'm often working outside an IDE.)

Including these types is more time consuming up front than languages that support type inference, or are dynamic, but for long-lived applications it seems like the overall tradeoff is usually worth it. To be fair, I haven't maintained any F# applications. If the type inference is purely saving annotations that are completely obvious even to a beginner to the codebase, then I could see that being worth it. (It seems to me that we are all beginners to the codebase in meaningfully large platforms.) The actual act of typing the annotations is also often pretty easy and handled automatically by the IDE, but I suppose you have that experience with C#.