The jewelry and investment industries make up 45-50% of gold consumption. Practical and industrial uses make up only 5-10%.

As such, while flooding the market with cheap gold would rapidly lower value, that's unlikely to be how the gold is sold. If the amount of gold being generated from fusion reactors is orders of magnitude less than the global consumption rate from jewelry and investment, which seems likely, and they are selling at our near market value rather than trying to undercut everyone, then the value of the generated gold would remain relatively stable.

in other words, considering that the gold market generates something like $350bn USD per year, and the total market value of "above ground" gold around $25tn USD, even if fusion reactors generate $1bn USD worth of gold it would have a negligible impact on the price of gold while providing significant value to the reactor operators (incentivizing the growth of the fusion reactor industry)

Alchemists (correctly) observed that everything in the world was subject to disorder and decay as time progressed, but noted that gold seemed to be immune to this effect (since it is highly resistant to oxidation). Add into that the belief system that they were working with:

• That everything in the world exists on a chain of being from the most corrupt at the bottom to the most noble on top (with god being most high).

• That everything is really the same thing, and through physical processes changes its form, including up and down the chain.

And they belived that if they could figure out how to transmute a lesser metal into a more noble one then they could probably move other things up the chain of being as well. Which is why the Philospher's Stone was supposed to make people unaging and immortal, and cure all disease, in addition to transmuting lesser metals into gold. Alchemists like John of Rupescissa probably belived that creating the Stone would also bring the world closer to the divine in some way, and it was god's wish for mortals to do this.