Riviera is supposed to be all DVC. Adding a new resort seems always to have some adverse impact on 7 month reservations at any near park resorts. Moreover, if, as I suspect, the nightly point requirements for Riviera rooms are going to be more than the same-sized BWV preferred and BCV rooms, you could see BWV and BCV rooms disappear at 7 months out somewhat quicker than they do now. Also, as far as "traffic" at BWV and BCV, it is not clear what impact Riviera is going to have but the existence of the gondola system should make it easier for all those in all the resorts in that area to come to the boardwalk or BCV restaurants at night. Likewise, you will have far more people using the International Gateway for entry into and exit from EPCOT.
But BCV's and BWV's problems with getting rooms using the 11 month reservation window are not likely to change in any significant extent because of Riviera. Riviera could possibly have some minor impact simply because making it harder to reserve things at 7 months out can lead more owners of a resort to reserve at 11 months out at their home resort instead of waiting; however, that kind of effect has likely already happened close to its fullest possible extent because of the increasing difficulties at 7 months out that have occurred in the last 8 or so years. The main causes of problems that exist for BCV and BWV, particularly for studios (and resulting 2BR lock-offs), during the 11 month window are not related to the additon of a new resort.
Other than for BWV standard view and on rare occassions boardwalk view, the two resorts have not really had issues booking at 11 or even 10 months out during the DVC low to moderate demand season that runs from the Monday after marathon weekend in January to late September. In fact, BWV pool/garden view is usually open even at 7 months out, and BCV likewise about 50% of that time, and you can even get lucky a number of times and get BWV boardwalk view at 7 months out. The hardest times to reserve during that season are race weekends and that is likely due both to the popularity of such weekends with DVC members and to a high amount of rentals for such weekends (a factor of the internet and rental brokers making it easy now to do a rental).
It is during DVC's high demand season, from late Sep to marathon weekend in Jan, where the ability to book BCV or BWV studios if you wait past 11 months out continues to get more difficult. The time you could not get, Dec 9-12, is part of the first two weeks in Dec, extremely high demand times (the first week is DVC's highest demand week of the year, the second week is the third highest -- in between the two at second is Christmas week). During those weeks there is even a real risk of not getting BWV standard and boardwalk view studios and to a lesser extent BCV studios at exactly 11 months. Other extremely high demand periods during that season include the Thursday before Columbus Day to the second weekend after Columbus Day, the Wine & Dine race weekend in the first part of Nov (the highest demand weekend of the year), the last weekend of Food & Wine in mid-Nov, the Tues to Fri of Thanksgiving week, and marathon weekend in Jan.
The causes of the high demand during that season for BWV and BCV (and other near park resorts) are several, all unrelated to having a new resort. The high demand season has some of the best weather in Orlando for the year. It then has all the events Disney has created for it -- Halloween, Food & Wine, Christmas decorations and events (which now starts in first half of Nov). It has the lowest and second lowest nightly point requirements except for three days around Thanksgiving and the last week of December. In essence, it is an ideal time of the year for DVC members to go. Added to that is the aging population of many DVC resorts, including BWV and BCV, where annually more and more of the earlier purchasers escape having to obey school schedules for their vacations because their kids are now grown. More and more of those want time during the high demand season and many of them no longer need a 1BR or 2BR but instead now want studios, and with the lower point requirements can take longer vacations in those studios than they ever did in 1BRs or 2BRs when they had kids. Disney and the internet also helped to increase demand for studios at 11 to 10 months out during that period. Disney by selling low (as low as 25 point) add-ons over the years, many of which have been sold to others by owners easily doing so by using the internet, and a huge number of those resale purchasers now have only a low number of points, which are to be used, ideally in their minds, for studios during that high demand but low point cost season. The internet just generally added to demand at 11 months out by providing information to owners where many learned that they could be blocked out if they did not book 11 months out. Moreover, after the Great Recession struck, Disney added to the problem by abandoning its minimum point requirements for new purchasers and creating 100 point minimums which at times became 50 point minimums, for sales of points to new purchasers, including at BCV and BWV that it got back through foreclosures or exercises of right of first refusal for resales, thus creating even more BWV and BCV owners who would want studios during that high demand season. Finally, the growth of the rental market on the internet has likely had some impact on demand at 11 months out during that high demand season, particularly for the November and January race weekends.