I speculated about the Sentinel development timeline before the movie came out, and I think I'm sticking to that: Sentinels were a black budget project for a while, and it was about to be cancelled when in '73 the project got a big boost from Trask's death and Mystique's capture. Still, something as complicated as those 2023 Sentinels would've been beyond the tech of the '70s, particularly when the most famous dangerous mutant controls metal, so metal can't be used in the construction at all. (How Trask built robots without transistors or circuit boards, just "space age polymers," is an epic handwave.) At some point, Mystique must have escaped, so that would have been a setback for the Sentinel project.
After Nixon was impeached and resigned, the Sentinel program fell out of favor-- it didn't have Trask to champion it anymore, and in the absence of any further major threats from mutants, the motivation to pursue it died, plus in the wake of Vietnam public support for militarization was low, and at that time the intelligence communities were undergoing scrutiny for abuse of resources and power... so many reasons why the Sentinel project might have been back-burnered.
It probably hung in there by a thread during Carter's presidency, but then Reagan, who never met a Pentagon boondoggle he didn't like, increased its funding, spurring active Sentinel development again; but it was still beyond the current tech, plus it wasn't a priority compared to Reagan-favorite programs like Star Wars. Under Clinton, the program probably would've been downgraded again, stalling the project.
Then, in 2000, Magneto launches his plot to turn influential humans into mutants, and is eventually arrested and imprisoned. Anti-mutant paranoia resurges and the Sentinel program is revived, and now the technology exists to actually realize the Mystique-inspired Sentinels, so they're being designed, built and tested from 2000-2006, when the original X-Men trilogy happened. Shortly after the battle over the mutant cure, the first Sentinels were unleashed and the dystopia begins.
Re: sentinel timeline
After Nixon was impeached and resigned, the Sentinel program fell out of favor-- it didn't have Trask to champion it anymore, and in the absence of any further major threats from mutants, the motivation to pursue it died, plus in the wake of Vietnam public support for militarization was low, and at that time the intelligence communities were undergoing scrutiny for abuse of resources and power... so many reasons why the Sentinel project might have been back-burnered.
It probably hung in there by a thread during Carter's presidency, but then Reagan, who never met a Pentagon boondoggle he didn't like, increased its funding, spurring active Sentinel development again; but it was still beyond the current tech, plus it wasn't a priority compared to Reagan-favorite programs like Star Wars. Under Clinton, the program probably would've been downgraded again, stalling the project.
Then, in 2000, Magneto launches his plot to turn influential humans into mutants, and is eventually arrested and imprisoned. Anti-mutant paranoia resurges and the Sentinel program is revived, and now the technology exists to actually realize the Mystique-inspired Sentinels, so they're being designed, built and tested from 2000-2006, when the original X-Men trilogy happened. Shortly after the battle over the mutant cure, the first Sentinels were unleashed and the dystopia begins.