To be fair, Mrazek might not be the goaltender everyone here thought he was. Howard has rebounded very well and still shows to be a decent level goaltender. FWIW, Mrazek has the worst numbers of all 3 goaltenders. The previous year where everyone was talking about buying out Howard and sending him for any return, Howard still had significantly better stats than Mrazek this year.
So the reality is, does Mrazek still have a lot of currency with other teams? And you can probably bet he does. So if you can get a decent Return on Mrazek, then you probably look at taking it. I dont see a reason to have both Howard/Mrazek. Coreau is a solid enough backup who might develop into a decent starter.
Then if Mrazek was just riding a wave of confidence and wont return to those numbers, you've dodged a bullet and potentially gotten a really good asset out of it.
If he does get good again, the numbers probably suggest it wont be much better than Howard and we still get a high end asset out of it and we are saving $5m in goaltender wages. In all likelyhood, getting that 1D/1C would likely have a bigger effect on the team than the difference between Howard/Mrazek anyway.
So I wouldnt be so quick to dismiss trading Mrazek. Especially given the numbers between the two this year show a significant play difference in Howards favour, yet Mrazek will still have a lot of trade currency.