Basically, you are suggesting that they don’t include popular characters that bring money in to the series so that you/we can be excited when they return. It is not really a great model for making money (NRS is a business not a charity). Leaving out Scorpion, Sub Zero, Raiden etc would make no sense as people are heavily connected / invested in them and they are important to the storyline. These characters sell games, which is why they include them in Injustice as DLC.
There are so many good games out there, why would people bother getting one that doesn’t have key things that they are interested in? The whole reason I never bought SFV was because Sakura / Blanka were not in it, so it was not particularly interesting to me in comparison to other games (yay season 3). If Harley wasn’t in Injustice, I would not have bothered buying it. Yes, there are many people like me and NRS knows it or they would do as you suggest.
You seem to be confusing top tier favorite characters with “Tier 2”. Tier 2 are popular without being iconic staples of the series. They are rarely massively important to the storyline and usually just act as minions.
Ermac (who is a favorite of mine), while popular, falls in to this category. He was never really a ridiculously popular / important character in comparison to Scorpion / Raiden etc, so missing out was never a big deal. Rain, Nightwolf, Baraka, Kabal, Smoke, Jade etc all fall in to this category – all have decent fan bases, but are expendable.
If you said leave out some of the popular ones and make them DLC (which will happen with Mileena), I can see more of your point, though you would need to at least have a good amount of the favorites or people won’t buy the game.
I personally think there should be a nice mix of the most popular with some more unexpected "tier 3" characters with updated storylines and looks eg Ashrah, Li Mei, Fujin, Havik, Hotaru etc