There may be other VFET manufacturers out there in the future. The VFET/SIT is pretty efficient for power switching applications and there is a huge focus on electrical energy storage and conversion for battery powered systems (Cars, Industry, Housing etc..) So keep your eyes out for new VFET/SIT products coming to market in the future.
The 2SK18x are sometimes reported to have highish gate current. I have not observed this with my THF51, and haven't read about this issue with THF51. If gate current is a thing with your amp, this might be relevant. Then again, those power SITs like a beefy driver, which also avoids any issues with gate current.
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People on the forum here have successfully built designs with both 180 / 182s and between Choky and Ben Mah, you are covered with what ever you have. If you can get THF-51s, then even better, but not necessary. Ben's choke loaded 2Sk180 is certainly one of his favorite amps, if not his most favorite.
I think I have a favorite but then whatever I have in the system sounds great. All of my SIT follower amps sound great. Currently I have my winter amp, the THF-51S mu follower follower, in the system and I am enjoying the sound. The weather is finally warming up so my spring amp, the 2SK180 choke loaded follower, will be playing music soon. 
If this was to me, the change is moving to sustainable energy. Switching DC voltages from batteries of storage cells requires some very efficient power transistors, so there is increasing demand. Now the toss up will be between GAs and VFET/SIT Silicon based designs.
In your comment, I'm assuming GA == GaN?
When I was in school, I was a lab technician in a nanotech & epitaxial lab. I got to characterize the GaN and other III-V semiconductor compounds grown on Si. It's neat to see that the industry has finally started to make a more serious shift towards that direction.
Still, it'd be very cool to see VFET designs make a serious comeback. Would it lower the value of these vintage, Japanese VFET products? Possibly; of which I am an owner.
But, it'd also provide a wide array of component selections to also repair these vintage designs which unobtanium devices (e.g., 2SK78).
A rising tide lifts all boats.
Yes, you are correct, I meant Gallium Nitride, but brain lag occurred.
I know of at least 1 serious semi manufactured that is/was looking at a new generation of VFET fir power switching.
In regards to devaluing the older JP transistors, I doubt it would be a significant, as they are still desired for keeping the classic 70's VFET amps running but would seriously lower the cost for the newer DIY amp designs.
Remember that a lot of the charm of the older JP amps is in their circuit design, not just the components used.
I know of at least 1 serious semi manufactured that is/was looking at a new generation of VFET fir power switching.
In regards to devaluing the older JP transistors, I doubt it would be a significant, as they are still desired for keeping the classic 70's VFET amps running but would seriously lower the cost for the newer DIY amp designs.
Remember that a lot of the charm of the older JP amps is in their circuit design, not just the components used.
Indeed
- I agree. There are many solid state high voltage applications now where Mosfets can simply die from the harsh environment. Also - Maybe the making of the die is complex with a high failure rate? It was 3-D printing sort of.
The mosfet market was then dominated with the low voltage (car/battery) based stuff, with Rds=on of dropping to negetive values almost. Now the same environment sees 400V as a normal. So switching requirements change.
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