Is a 51% attack very likely to be unprofitable or simply risky but potentially profitable? Considering that we can fork and slash.

I never fully understood. If a miner had 51% of all eth and could thus double-spend he'd get slashed after doing so and would thus lose all the tokens. He cannot double-spend the tokens he has staked right? So whatever else he has is smaller than the 51% he'd lose, is that the idea?

And is the idea that a fork would need to happen simply before the miner could withdraw his stake basically?