Honestly when you think about it, there are sooooooooooooooo many mechanics in this game to make you buy things or watch ads that I really wonder if I'm not far from the truth...
If they really wanted to improve the game and add some content, they would have add worlds from the chinese version, with new zombies, new plants...they even have new zombies in the worlds that are in the international version !
(Btw the chinese version have more plants that the international version but only 3 premium plant !!)
All the mechanics in can think about:
1. All the upgrades that you have to buy to get bonuses inside and outside the game (power bundles and Pinata tracker)
2 .All the gemium plants that you have to pay with gems that you have with ads/quests/cash
3. All the premium plants (Almost 1/3 of the plants of the game are premium...)
4. The gemium and premium plants that are here for a short time, so player are more susceptible to buy them since they don't know when they will be available again
5. Watching ads to get "free" gems" every day
6. Watching ads to get "free" gauntlets every day
7. Watching ad if you fail in daily Pinata challenge (most of the time the level are easy but they can be really tricky sometimes)
8 .Having the possibilty to boost some plants in exchange of some gems
9. All the special offers to buy premium plants
10. All the ads about thoses premium plants offers
11. All the special offers to buy special pinatas each week (depending of the plant of the week in Battle)
12. All the ads about thoses pinatas offers
13. Having to buy Pinatas with gems (so again cash/quest or ads) if you want to upgrade a specific plants (else it will take forever with the Pinatas and seed you can get daily)
14. Buying Pinatas with cash
15. Pinata system by itself which is in fact like a loot-box, it is totally random
16. Having to pay with coins (so ads or cash) to upgrade your plants even if you already spent lot of time and gems to get the seeds
17. The price of upgrading that are just crazy at some point ! (seriously 20k to upgrade Hot potatoe ?!?)
18. Paying 3 daily pack of seeds with coins (= 30 ads watched)
19. Getting a lot of seeds of plants players don't have which makes player more susceptible to buy them
20. Some extra hard level that can't be finished without any kind of boost or power-ups (seriously how many coins players spent in total on Wild West Day 35 ? It is impossible to finish it without power-ups or boosted planted except if you are very lucky)
21. PvP system which by itself introduces a bias between FTP and PTW players and make players more susceptible of buying premium plants
22. PvP league system which force players to use gauntlets to go to the top and so either to buy them with gems (ads/quests/cash) or to watch ads
23. Some games setup that are unfair for FTP players, like banning almost all the sun producing plant but allowing Gold bloom, banning chard/blover combo but allowing cauli.
24. Posibility to keep your streak in exchange of gems
25. The infamous 10th game streak breaker that breaks many streaks and either make players get less reward or players pay to keep their streak
26. The plant of the week which is amost all the time a premium plants or sometimes a gemium plant which make players more susceptible to buy it
27. The weekly quest for the plant of the week that will give you seeds, so again making players more susceptible to buy it
28. Adding of plants (power-mints) that can be obtain only by the PvP sytem (so see all the things related to the PvP system
29. Price of power-mints and of their seeds that are really expensive, well except if you are a streak expert like @Humplemann (which recquired skill and a looot of gems to keep your streak)
30. Power-mint and related seeds that are limited in time making player more susceptible to buy them
31. Possibility to buy costumes for plants in exchange of gems
32. No longer gems dropping randomly in level like it used to be in the beginning
33. No longer Treasure Yeti who used to give coins (1000) or a costume when defaited
34. No longer Epic Quest with premium plant as a reward
And I may have forget about some...
There were already stuffs to buy when the game was released (I think that each power bundle was around 5 €-$-£, now it's 5 for all of them), but with every updates, I think they added new mechanics to make people watch ads or pay premium or gemium plants and meanwhile reduced mechanics that made players getting things for free (see 32,33 and 34)