Because medicare expansion might very well be worse than your employer plans?
There's a LOT of different strata to the US employer health insurance market. Union plans are part of it, but there's plenty of middle class white collar workers with decent coverage.
The thing about the costs argument is that right now, the costs are hidden pretty effectively. A Medicare for All system that totally supplants the current structure puts all that out in the open and people get sticker shock.
Plenty of Democrats can get behind single payer, even more support a public option and other addons to the ACA like dropping the Medicare age to 55 and, yeah, offering catastrophic plans for people young enough to think going to the doctor is mostly pointless. This whole "Dems have no ideas" crap is silly. We have a ton of ideas. We don't have lockstep consensus but that's because we're interested in figuring out which idea is best, as opposed to Republicans, who have exactly 3 real policy goals: reinforce the patriarchy, reinforce white supremacy, cut taxes. It's gonna take us longer to get all moving in one direction, and the debate around ideas is a healthy part of that process.