When millionaires need and qualify for subsidies then there is a problem. The problem isn't that our MAGI is too low. It's that the cost is too high.
With a progressive tax system, there is always the risk of demotivating people to stop working because with each dollar earned, more is given to the government. And policy makers typically look at the 50% effective tax rate including federal, state, and local as the tipping point.
With the ACA, I think this will start happening at lower dollar figures where a couple who make $100k a year will find it more appealing economically for one spouse to stay home, so that they can keep ACA premium subsidies.
Of course, if a significant number starts pursuing this strategy due to these perverse incentives (similar to how this thread is starting to expose), premium subsidies will ever increase, and the government will need to find more sources of taxation.
They will likely go after the 250k+ income earners or perhaps they'll start looking at things like wealth tax or perhaps Medicare for all? This will be an interesting ride.