IIRC, the sum total of all fusion research throughout all of history is USD$100-200B. It's obvious governments/industry/humanity doesn't really want it, or they'd go fund it.
The lack of funding angle isn't really convincing.
Modern designs depend on material science and computing abilities which could never have been made in the 70's no matter how much money was thrown at it.
Fusion-relevant materials research could have absolutely advanced with funding back in the 70s. Lithium compatible structural material and 14MeV neutron source experiments immediately come to mind, not to mention tritium permeation and extraction. There was tonnes to learn, and they chose not to fund it.
You could put teleportation on that bottom squiggle as well. Sometimes you stop funding things because it isn't physically possible or we have better alternatives.
There's absolutely no reason to believe that fusion power is impossible. It being very, very difficult and very, very expensive to reach a practical system is the problem.
But you're right about better alternatives. Right now that alterative is a combination of solar, wind, hydro and storage, with perhaps a bit of more exotic systems (wave power? solar towers?) in the mix.
Modern designs depend on material science and computing abilities which could never have been made in the 70's no matter how much money was thrown at it.