I don't think they ever intended Wii Sports Tennis to be the ultimate tennis game, for instance. What they're probably hoping is that some third party will come along, rip off the swing mechanics whole-sale, add in nunchuck character movement and all the bells and whistles expected in a realistic tennis sim (different types of courts, full length matches, character growth, career mode, etc.) and release a fantastic complete tennis package for the Wii. That makes Wii Sports Tennis (and the other Sports games) an advertisement for other full-fledged $50 games.
And if third parties don't take advantage then Nintendo will eventually release Mario Tennis Wii that does all those things anyway.