You point out that alternative reactions like D-3He are theoretically possible. But my argument is not that D-T (Deuterium-Tritium) fusion is theoretically imp

You point out that alternative reactions like D-3He are theoretically possible. But my argument is not that D-T (Deuterium-Tritium) fusion is theoretically impossible. It is practically impossible. While it is possible to generate very small amounts of Tritium in commercial reactors, a fusion power plant would need much much larger quantities before it could even be started. And while the D-D reaction is also theoretically possible, it is even more difficult to achieve practically. This is similar to aviation in 1800: While it was theoretically possible to build a flying machine, there was, for example, no known engine which would have been able to power an airplane. And for somebody to say "we just go ahead and will solve the technical problem of having a low-weight power source within 20 years" would have been delusional. You can't summon technological break-throughs like a mythical ghost in a bottle. So to sum up, your argument against that todays nuclear fusion research, which is dominantly based on the D-T reaction, will not lead to any energy reaction in foreseeable time, is that there are more exotic, more difficult, end less well understood reactions which are also theoretically possible but not even close to being practically usable in any way.