First, the reservoir won't empty completely. Second, if it were there will be meters of mud on the bottom of it. It's possibly more impassable when empty.
In short term yeah, it will probably be too muddy. But it will dry over time and in the long term this unlocked a completely new axis of attack for Ukrainians. Similarly in the downstream regions. The water will go away eventually and it will be much easier for Ukrainians to attack in the future, when the Russians will not have the capability to flood the downstream regions with this dam.
The Dnipro is still a large river to cross and it's not going away. I looked at it in Google Maps and it's at least 500 m wide before and after the reservoir.
I am actually not sure how wide it will end up. Dnipro is actually reservoirs all the way up to Kyiv. It will be Ukrainians in Dniper HES that will control the flow from now on.
