The grid is for trade, so I am not sure what sense this complaint makes. Also France relies on imports sometimes. Electricity prices changing with production and demand is also exactly what market is for. As long as the market is working, this is beneficial to buyer and seller and changing prices are signals that allow the market to optimize production and consumption. Even when the price becomes zero sometimes, that does not mean than renewables need more subsidies if they earn money at other times where the price is higher. Also Germany still has enough of conventional generation capacity to ramp up production if needed, so the "is possible at all because" comment is wrong. If there is trade, then because this is cheaper overall (and in general this helps buyer and seller).