There's a difference between introducing new models and introducing new platforms.

Yes, Nokia comes out with new phone models every few weeks, but they only change software platforms every few years. Nokia smartphones sold in 2006 can use exactly the same applications as Nokia smartphones sold in 2007 and 2008, because they all run the same S60 3rd Edition platform.

As long as the new tablets all use the same software platform and are binary compatible, I don't see why there should be any limits on how many models they bring out.