It took me quite an effort, but I finally got comfortable creating music on a computer, so I totally get now that young people will most likely buy a $100 MIDI keyboard, download (buy) some software and will proceed this way.

The companies like Yamaha see it as well. In fact, Korg hasn't replaced their PA1000 for 7 years now, Roland is out of the game; Casio introduced their CT-X models years ago, so SX920 doesn't really compete with other arrangers.

Although PA1000 is still a competetor, but an old one. SX920 sounds quite similar to SX900, and SX900 is not so different from S970, and so on, but small improvements gradually lead to a major difference between, let's say, PSR-3000 and SX920, although as for physical quality, I witness quite a regression - buttons on my S950 are unresponsive, keys on my SX900 are noisy and service doesn't fix everything. PSR-3000, meanwhile, works fine (changed contact strips).

There are two reasons why arranges aren't exactly for younger music (EDM, trap, hip-hop, etc). The sound itself is actually a second one: it's just not arrgessive/dark/solid enough for these types of music, besides this people always keep looking for new ones. The first reason it that this type of music is based on MIDI editing, not recording audio signal from your keyboard. All the notes have perfect timing (quantized), otherwise this type of music won't work. Yes, you can connect you PSR and use it as a sound engine, but it doesn't worth it.

There is a nuance, though, it is that not all young people actually like young music, and limiting yourself only to working with a computer means missing a great deal of emotions which playing an arranger can provide, but all in all I think Yamaha is being pragmatic by keeping PSR the way it is, they are willing to lose some battles in order to keep winning certain others.