I dunno the X3000 still feels like very modern scope, it even has a touchscreen. All the other low cost scopes ware just playing catch-up to it for a good few years when it was released. Now that they have caught up Keysights low end scope lineup is on par with everyone else.
But even when the successor to the X3000 launches i wouldn't expect there to be that much new. I still use a old MSO6000 that is pretty much the predecessor to the X3000 series. The two scopes are very similar in operation, UI and performance. What you get with the X3000 is higher waveform update rate, waveform generator, touchscreen, some modernization, but at the same time the 6000 has 4 times as much waveform memory, 1/5 the boot time, all interfaces available without a add-on module (USB Ethernet GPIB), higher resolution screen etc...
The now discontinued MSO6000 series is so old that the predecessor to that still had a CRT screen and carried a Hewlett Packard logo. So id say we have at least 5 years or not more before Keysight would consider updating it.
Scopes don't age anywhere near as quick as consumer electronics like TVs