First, make sure you are really to the wear indicators, and not just the higher water-block bars. The wear indicators are located wher the tiny triangle marks are on the sidewall. 8,000 is not much, but it isn't unheard of. 10,000-14,000 seems to be average. The tire wear depends a lot on riding style, road conditions, and especially on speed. The center will definitely wear faster at freeway speed limits or greater. This kind of tire wear is not unusual for a motorcycle, although it is substandard for an automotive type tire. A typical touring bike seldom sees more than 10,000 miles on a tire, and 5,000 is not unusual. The OEM tire is kind of a hybrid between automotive and motorcycle tires, having an automotive shape, but being softer for traction purposes and being subject to more wear and force due to the three wheels, and subject to side forces from uneven loading and the leaning forces in the turns.
Changing the tire cannot cause cancellation of your warranty, and can only cause warranty repairs to be rejected if it can be shown the tire caused or contributed to the failure. That is the law, although many dealers will try to bully you into thinking otherwise. The tire brand isn't necessarily important, but the tire diameter is. It has to be close to the original size or Miss Nanny can get angry. The tire tread pattern, speed rating, and wet and dry traction are also important factors. Some dealers will not change a tire other than OEM.
Unfortunately no manufacturer has stepped in to make an equivalent tire, although there are some acceptable automotive substitutes. The choice is yours...but don't expect 50,000 miles like you would on your car.