I am also considering a European river boat cruise for next summer.
The most poignant factor I've come across in my research is that THE RIVER CONDITIONS are a huge, unpredictable factor in a Euro river cruise. If the rivers are too high (as in Spring), some of the boats won't clear under some bridges, and if the river is too low (mid-summer), some of the boats can't navigate. (typical draft is 5-6 feet). Apparently they don't have the sophisticated system of locks & dams that we do here, to keep the rivers navigable (although they do have some locks and dams).
What the cruise lines do, generally, is to convert to a motor-coach tour, without refunds or any special consideration for the future. Sometimes this affects one or two stops, and sometimes it affects the entire cruise. The cruise lines never mention this factor, although between 10-15% of all river cruises in Europe are negatively impacted by river conditions. Obviously, it would be a big disappointment, and would totally void the main reason why you are choosing a boat rather than a motor-coach tour: not having to re-pack your bags every day.
Viking is the "Chevrolet" of Euro river cruising, with others being more and less opulent, depending on price. Viking wants payment up front for next year's cruises, which leads me to believe they are cash-poor. They have been building new boats with reckless abandon for several years, so that may be why.
I will shortly be in a position to take advantage of "last minute" fares and discounts, as both my wife and I are planning to be fully retired by April 1st. But every time I follow one of those ads or links, it goes up in smoke, and you are looking at the same rates for the next cruise as they are asking for a cruise a year from now. I intend to keep trying.