all products, decking and flooring, that go over a substrate like concrete or any other surface always do better when that substrate is level, clean, and dry i.e. no standing water . seeing to the substrate can be a preventative step in protecting your deck.
if done properly, new concrete can often be poured right over an existing slab. for this to be feasible, the contractor needs to pour at least 2 inches thick, use smaller aggregate, and incorporate reinforcement such as welded wire mesh or fiber mixed into the concrete.
price per square foot in my experience is about $5 - concrete, $8 - stamped concrete, $12 pavers. decking should be about about the same as the pavers or more depending of the materials you choose. it seems as though you are comparing installed prices for concrete/pavers with material costs for decking.
if the pressure washing damages the concrete matrix, you should tear out the deck and build a new one. if a few aggregate particles come off but the matrix is sound, proceed with the overlay. start by applying a degreaser with a pump sprayer to remove oils, such as those from spilled foods or suntan lotions.
a loose packer here when there's a tight packer elsewhere, over the distance of the deck will add up if you don't keep checking. or do as some do and lay intermediate decking pieces perfectly strht and square to your perfect subfloor and fill in between with rest of decking, using lots of wedges or packers.
cleaning and preparation. before attempting to pour over existing concrete, remove stains and loose material. put on safety glasses and use a hammer and chisel to chip out oil stains or tree sap. if the stain covers a large area, cover it with a sealer to prevent problems with bonding when you pour the new concrete.