Welcome to the would of modern bikes, you must have got 4400 in it's last year, there has also been a lot of competition between the big 3 players in groupsets for the last 5-10 years increasing the number of gears, electronics, the use of Carbon Fiber etc, along with built in obsolescence.
Saying you can't get spares is incorrect though, as you can normally get Shimano spares for as long as they are listed on the Tech Docs, which 4400 still is, but this will be for individual parts, not complete units like an RD.