The 3 episode split was the best idea in the history of the franchise. This game has so much SP content thanks to that decision, it is shocking considering how it is mainly a MP game. I just finished replaying WoL for the third time and HotS for the second, LotV is still too fresh for me to replay, I've already finished it once.
I disagree, SC1 had about 10 episodes per race, and BW added the same. While SC2 offers a lot more side options, like upgrading how you want, it doesn't actually have a lot of content. Most of the missions were designed at once, which explains why Void rays still charge up in Legacy of the Void, even though that was changed almost 2 years before LoV was released. The mission design is a bit more discouraging to me, it still matches the 2/3 base max that Wings of Liberty started out as. The first BW mission had 3 bases available, and more if you wanted to take the enemies.
I still love the game, I just think they should have designed the single player missions so that multiplayer was easier to get into. There's very little correlation between single and multiplayer with many units having completely different functions.