Funding was reduced from those projections because they were for from a crash project that assumed tokamaks would work really well, much better than it turned out they did.
And in hindsight, even given optimistic plasma assumptions, fusion reactors would have been limited by materials and heat transfer issues. These render any DT-burning reactor problematic, independent of the details of confinement.
Yes, the fundamental idea that $X spent will result in Y scientific knowledge and Z technological advances is managerialist wish-fulfillment. Science and engineering aren't remotely that predictable. We can imagine that we know what we need to discover in order to accomplish something, but the universe likes to laugh at us.
The talking-point that keeps coming up about fusion spending ignores the context that the planned spending was based on a technology that wouldn't have gotten us practical fusion, anyway.
If that $X had been spent, fusion boosters would be complaining about the debacle of wasting all that money on tokamaks and how that undermined fusion research. And if we spent a newly-calculated $X and, years later, still didn't get fusion, the complaint would be exactly that managerial whinge that we just hadn't shoveled enough money down the hole (or down the right hole).
And in hindsight, even given optimistic plasma assumptions, fusion reactors would have been limited by materials and heat transfer issues. These render any DT-burning reactor problematic, independent of the details of confinement.