I just don't think these are fair comparisons. Smash has such a huge number of people working on it-I've just looked it up, and Smash credit
107 people for working on character animation and modelling alone. According to Wikipedia, the total number of people Next Level Games employ is 117
And as for Mario Tennis on the N64, different eras, different resource requirements, different objectives in the game's development. I imagine it was far easier to tweak base models into new characters back on the N64. In Strikers, there's a lot more going on unique to each character too-they need gear moulded specifically to their unique models, the super strike animations etc.
Edit: The Gamecube entry only had 8 captains and 4 side kicks, so 10+10 is comfortably an improvement on that. The Wii had 12 captains and 8 side kicks so I guess it's more comparable to that but with the "captains" now taking every position.
I guess the question is would you rather have the game later so they can finish all the characters and release the full package or have it June 10th and have new content released in the following months. I think most people would prefer the latter but appreciate it comes down to personal choice. You can also achieve the first by just buying it later