This S-Pen is very specific to the Fold 3. I tried it on my Tab S7+ and it barely works, it's erratic. And if you try another S-Pen on the Fold 3, a popup will appear before you can even touch the display, basically saying to use the S-Pen design for it, and it will not work. There's also a spring inside this S-Pen that so the (very soft) tip can slightly retract and extend to prevent poking/scratching the display.
So being that said:
Any possible future 3rd party S-pen that's approved by Samsung, will be much more than $40.
Any possible future 3rd party S-pen that's NOT approved by Samsung, will most likely damage the display.
Now tell me which company is going to go through that headache for a $40 or less stylus that's only compatible for one device?