While this plan might sound good in theory, the logic doesn't take long to unravel.
D'Antoni is 69 years old. When negotiating an extension with the Houston Rockets last May, he said he wanted to coach two or three more seasons—which included 2019-20 at the time.
Even if D'Antoni is reinvigorated and wants to prove himself deeper into his 70s, he isn't in a position to wait around two years on the possibility someone might follow him to Philly.
Harden will also be 33 years old by the time the 2022-23 season begins. The 2017-18 NBA MVP presumably won't be washed up by then, but he'll be at or near the end of his prime. The idea of handing him more than $50 million per season over the course of his mid-30s—which is likely what it would cost, though that will depend on the NBA's financial situation—seems like a stretch.
The Sixers also aren't projected to have salary-cap space in the 2022 offseason. They could trade for Harden before then, but that would likely require moving Ben Simmons or Joel Embiid along with other assets to make the financials work.
Wilder things have happened in the NBA, but the odds of this coming to fruition are beyond slim.