I've heard that RTB / programmatic means less valuable ad inventory and cheap sites. Is that true?

That's incorrect. Real Time Bidding, or "programmatic" is a more efficient way for buyers and sellers to transact. It does not imply any particular type of ad inventory or even pricing model. In open selling, anyone may bid and buy inventory without a prior deal or relationship with the seller. The seller may still, upon receiving bids, inspect and manually audit the buyer and the creatives. In private deals, the seller and buyer has made a pre-negotiated deal to buy and sell ad inventory at a specific price, just like any old transaction.