Look at any nation that underwent major coups; factions form, and it tears the organizations you've listed apart at the seams. Because a conflict of legitimacy exposes those seams, and those seams are absolutely present today. A Secret Service agent, an army colonel, armed militia, border patrol agents—if they can be made to believe the results of the election are illegitimate, they may consider the best way to fulfill their oaths to be stopping the "illegitimate" president from taking office. They will think of themselves as the ones stopping the coup.
I'd love to believe that all of those dynamics you're describing are the same as they were 20 years ago. I'd also make your argument then. But they aren't anymore. It can happen here.
Trump now though? Nobody fears him, the majority disrespect him, government bureaucracy openly defies him. He doesn’t even have the House, nor enough Republican support to pass laws to enable a power grab, nor a Supreme Court loyal to him before the constitution.
It's no-longer a question of if the Democrats respond, it's a question of how.
Actual court packing is a terrible idea, last attempted by FDR, at great political cost. Hearing candidates actively propose plans for doing so boggled my mind.
Maybe not so much. Didn't this court gut important parts of the the Voting Rights Act?