That is a very dismissive view of non Linn alternatives imho.
Stack Audio's Alto and particularly Soprano Sub-chassis I suspect are serious products at price points that might appeal to many LP12 owners who do not wish to afford Linn upgrade prices.
I can only offer my own experience on this. During the first lock down, as I was bored and wanted a project, I ordered Stack's previous subchassis, a £250 rigid, composite and well finished device. I felt it was not too much financial risk. I have watched my LP12 being serviced by several pros so I felt confident to give it a go.
Out came my Cirkus chassis and I attached my Cirkus bearing to the Stack. I had little or no expectations as to how it would sound.
Frankly, I was staggered by the increase in detail and low level information, and playing every record I knew really well threw up surprises. It was just better in every way. In the autumn I put the Cirkus chassis back in for half a day. Sounded terrible in comparison.
I have heard a Kore and a Keel, but not in a comparison situation. I would be surprised if a Kore sounded as good, different, Yes, all products will sound slightly different. I would imagine the newer Stack products are better than my older one.
I have probably played more vinyl in the last year than for years. Next stop, probably a Karousel.
I have a Naim Aro arm, probable an LP12s most famous 3rd party upgrade/alternative, still many peoples first choice, if you can find one. I've never heard anyone dismiss that because it is not made by Linn.
Just my thoughts.