Backwards compatibility is great and all, but at what cost? One of the main reasons why Windows eventually started to collapse under its own weight was backwards compatibility.

If a piece of software is so tied to backwards compatibility that it cannot change it will, necessarily, die.

That said, statements like "Once added, you can't ever make any changes to lambdas" are obviously bullshit. Programming languages can and do successfully undergo significant changes. Ruby 1.9.2 is a completely different beast than 1.8.0 (including significant changes to lambda sytax), and I'd expect there are few (if any) applications that could be upgraded without significant changes. The same could be said for PHP 4 -> PHP 5, Perl 5 -> Perl 6, and Python 2 -> 3. Contrast those languages to Java, which is largely the same since 2001.

Sure, there's a cost to everybody when you have to upgrade your app, but there's a much greater benefit when it is all said and done.