first: we would be completely reliant on German economy
...and vice-versa, Germany would have to rely on our supplies.
second: as a suppliers of components we would receive much less income
...but we don't supply components only. Germany buy our trains and trams, for example.
Also, you have to be realistic - we supply components for German car industry, simply because we don't have our own, and it is highly unlikely to create automotive industry in Poland able to create Polish brands comparable to Audi, VW or Mercedes and able to compete with German products. So, we do what we can in the current circumstances.
The state is not as effective as a private owner when it comes to managing companies.
That is absolutely correct.
However, when it comes to strategic sectors you cannot place them in the hands of individuals who can buy/sell them to anybody at will. I am too old, and have seen to many things in this country, to trust the ownership of strategic sectors to Januszes in white socks (or even those in black socks).
energy and military have to comply with state rules
Energy and military, sure; banking, media and industry, not so much (at least controlling them is much more difficult). And even in the energy sector, if you put it in private hands, you risk actions which go against national interest of Poland.
Some things have to
be managed centrally, by the government, because even local authorities
, being theoretically state entities, can act against Polish interest. You will find a good example here...
... a strategic sector company entrusted to local authorities (the very same thing would/could happen if it was privately owned).
So, as I said - some sectors have to be centrally state-owned, without EVER entrusting them to either private capital or local authorities.