Also, we've been watching eight man teams in one-week races for aeons. It will just be like that until fatigue kicks in in the second half of the race, when things become less controllable, because there are fewer domestiques, attrition will cause riders to drop out because there are fewer riders to share the workload with, so while yes, the chances of breakaways winning stages become higher, this only incentivizes more riders to get into those breaks, and makes it harder for the big teams to stop strong opponents getting into those breaks. It also means that team leaders may be protected less, so are subsequently more vulnerable to attacks.
Think of how so many of the big exciting stages of recent years have been caused by a lack of control as a team's weakness broke the race up. Heras stuck three or four men up the road, but if Menchov had four guys to pace him through the valley, it would have all been for nothing. If Caisse d'Épargne had more men left in the race, Landis probably never gets allowed up the road in the first place. L'Aquila and the entirety of the second half of the 2010 Giro doesn't happen if BMC's support squad wasn't so weak and/or inexperienced that Evans couldn't rely on them and the terrible weather didn't make some of the Astana domestiques get sick so Vino had too few people to help him deal with the number of escape attempts to the point where both teams lost several riders that day and Vino and Cuddles were taking pulls while the likes of Arroyo and Sastre still had helpers up the road. Fuente Dé doesn't happen if Катюша didn't have so many tired legs from defending Purito's lead for two weeks meaning they disintegrated around him when Contador made his move. Formigal doesn't happen if Froome has three or four helpers, because they'll start pulling the gap back down, then Tinkoff and Movistar will desist and leave it to the final few kilometres.
Look, if it doesn't work, they can always just reinstate the extra numbers next year, but for years we've been talking about how races have been more exciting when there's less control due to lower numbers, e.g. the Olympic Road Races with 5 per team max, or races that provide good racing with 6 man teams that would be far too easily controlled with 8 man teams, like the Tour of Britain. The reduction in team sizes has for years been a suggested measure to try to combat the overly formulaic racing in the biggest events. Hopefully it works as we would like.
However, if it doesn't come with a simultaneous downsizing of the race entourage it may be for nought. Fewer cars, motos and smaller caravans would enable a lot of interesting locations to come back into the reckoning for races that presently they are not able to accommodate, for example, but also I do have to say that this would be a wonderfully typical cycling response to the moto problem.
At the Vuelta, they noted their mountain stages were producing smaller gaps. So instead of designing better mountain stages that would produce bigger gaps, or introducing enough time trial mileage to incentivize harder racing in those mountain stages, they just figured "do the same mountain stages, just do more of them". Similarly it wouldn't surprise me if the race organizers' reasoning is "it's getting crowded on the road with so many riders and motos and it's resulting in a dangerous situation where the motos can't get past the péloton and are hitting riders. Therefore, we should make sure the motos have enough room by reducing the number of riders"...