What makes you think that? The torrent protocol would need to fundamentally change to completely stop the older versions from working. I don't see that happening.
The environment it runs in (your PC, operating system) changes over time. Yes, backwards compatibility is a big priority, but eventually something is going to fall out of compatibility. It's also known as bit rot.
The last good version of uTorrent actually fails to run under Windows 8 64-bit for me.
I'm running uTorrent 2.2.1 on Windows 10 Tech Preview 64-bit just fine.
And when it does, I can switch. For now, the older version of uTorrent works quite well and is RECOMMENDED by the trackers I use.
I'd love to hear a rational explanation from a private tracker operator as to why those recommendations are made. I understand banning misbehaving clients but the vast majority are fine.