You're pretty much right, but its not as bad as you think. Yes you do it all just for you. Tho with the friend thing you can "Share" your guys with other ppl to use. But it is just building up your team based on there skills and power to achieve the team you want. There are a lot of different play styles which is achieved by your team. But really its just about building up your team and progressing. Thats what is fun to me.
As far as spending money goes. You do not have to spend anything. There are a lot of non-IAP'ers who have really good teams and are really far. Its all about the events. There are lots of events that will give out a magic stone a day for say 7 days (one is starting on friday see
http://www.puzzledragonx.com/). These plus the stones you get for clearing the dungeons give you a lot. You need to use a decent amount on your bags but you will have enough to roll for gods. You want to roll during a godfest event. This gives the listed gods 3x drop rate and you will get a decent amount of them. I think we all spent some money so far but it doesnt have to be the constant money sink that you're thinking of.
The main goal is the special dungeons. They will have special ones that last a day or a week with a rare drop in them. Normally these are really good pets and can't be bought. Once you get a powerful enough team then you can do these. Some of these will even have 100% drop rate if you can complete the instance. They are really good.
It really comes down to stamina management. You get 144 stm every day if you dont let yourself get maxed. Sure people who just spend money can keep refreshing it but you dont need to. When an event is coming up you will get yourself almost a full bar of exp. Let your stm fill up. Then you can burn your stm in it have the exp lvl you and then burn it again. You can do them quite a bit without even using $$. And until you get high lvl. (im lv111 and can still level every day without using stones) it wont really be an issue.
I think if it did have PvP in it then you would def want to be spending more. As it is now you can play the game how you want. If you want to drop money, you can blow through it and have this awesome ass team. Or you can not spend that much. Sure it will be slower but as they say. Its about the journey not the destination.