How to reverse obfuscation in .NET?

No, it is about a lot more, especially with more sophisticated obfuscators. They can produce IL that cannot be expressed in most languages, and where the logic flow is horribly tangled to befuddle the best of tools. With lots of time you can do it (probably lots by hand), and there is certainly an arms race between the obfuscators and deobfuscators - but you vastly underestimate the technology here.

Also, note that many obfuscators look at an entire application (not just one assembly), so they can change the public API too.

That is certainly the start of an obfuscator. Though some obfuscators will also encrypt strings and other such tricks to make it very difficult to reverse engineer the assembly.

Of course, since the runtime needs to run the assembly after all of this, it is possible for a determined hacker to reverse engineer it :)

There are 'deobfuscator' tools to undo several obfuscation techniques like Decrypt strings, Remove proxy methods, Devirtualize virtualized code, Remove anti-debug code, Remove junk classes, Restore the types of method parameters and fields and more...

One very powerful tool is de4dot.

But there are more.