If the Stellerator, or any of its descendants, turn out to be the answer to practical fusion, it would have been contingent on vast quantities of computational power being available cheaply. It does not matter how much money you poured into your Stellerator project in 1970, you could never have built the 7-X, because there would have been no way to produce that design in the first place.
Personally, I think one of the answers to "tech seems to have stalled" is that it's easy to underestimate how important cheap computational power is, even without "AI", to moving forward. For another example, as fantastic as the 1960s space projects may have been, I think they just weren't economically sustainable, so it isn't that amazing that they didn't become a self-sustaining industry right away. I think it's easy to look at SpaceX and say "Gee, there's nothing that we couldn't have done there fifty years ago", but again, I suspect you miss just how much of the Falcon rocket is a result of extensive cheap computation abilities. Even if you could use modern computers to produce a design that could have worked fifty years ago, there's still not necessarily a practical path to that design using only tech from fifty years ago. (And of course the Falcon is full of stuff that couldn't exist fifty years ago.)
Modern tokamak designs are of course run through all kinds of computations nowadays too (because everything is), but the stellerator design, in a deep and fundamental way, simply isn't possible without massive computational power, whereas we've been building tokamaks since before massive cheap computational power.
Personally, I think one of the answers to "tech seems to have stalled" is that it's easy to underestimate how important cheap computational power is, even without "AI", to moving forward. For another example, as fantastic as the 1960s space projects may have been, I think they just weren't economically sustainable, so it isn't that amazing that they didn't become a self-sustaining industry right away. I think it's easy to look at SpaceX and say "Gee, there's nothing that we couldn't have done there fifty years ago", but again, I suspect you miss just how much of the Falcon rocket is a result of extensive cheap computation abilities. Even if you could use modern computers to produce a design that could have worked fifty years ago, there's still not necessarily a practical path to that design using only tech from fifty years ago. (And of course the Falcon is full of stuff that couldn't exist fifty years ago.)
Modern tokamak designs are of course run through all kinds of computations nowadays too (because everything is), but the stellerator design, in a deep and fundamental way, simply isn't possible without massive computational power, whereas we've been building tokamaks since before massive cheap computational power.