Compare this to a traditional game like Cities: Skylines. You pay once for the base game, then there are various DLC expansions that work as you suggest. You don't need them to complete the game, they just add new features and can only be purchased once.
Edit: 'Pay to win' is a better description for most games in this category (and Clicker Heroes 1 looks to follow that), although there are some free to play games like Dota 2, which have DLC that just provides cosmetic changes and isn't needed to win.
And for multiplayer just 1 player (I think the host) has to have the DLCs, everyone else gets them for free.
And BTW, Cities: Skylines is distributed by Paradox :)
This means that if you set a limit on it, you'd be limiting your profit by an extreme amount.
I'm all for it by the way, I think games that sell game affecting content for thousands of dollars are targeted scams portraying as games to get away with it.
Especially because games like league of legends and path of exile had shown how you can do free2play while only selling cosmetic items, if you make a good game.
Some people, like me, spent a ton of real money to unlock all champions faster. According to Riot, it's not "necessary" to have all champions unlocked. However if you don't own a champion, you cannot pick that champion early in the team drafting phase for someone else to play (swap/trade) - even in ranked. It is a disadvantage, though admittedly minor - a 5 man team that owns all champions has more flexibility in drafting than an enemy team that has holes in what they own.
With enough play times, you'll have more than enough to buy all the heroes without paying real money and then there's nothing else you can use it for.
Real money is only required if you want to buy skins.
Also, with hundreds of available champion to play you are not really required to play the most recently released champion. Just play from pools of all existing champions until you get enough point to buy new champion.
Simulators are different. You do not gain competitive advantage by buying DLC. You get some more content, and usually they're high quality, historically accurate and come with nice documentation (checklists, etc).
No-one would have the time to play with all the DLC in train simulator. There is hours worth of playing in each. Heck, it took me an hour just to figure out how to get a steam locomotive out of the rail yard.
Flight simulators and racing sims (iRacing) are similar. You can expect to spend hours of time with a new aircraft or race car. In racing, you race against others with the same or similar car, so you gain no advantage.
I can warmly recommend trying out Train Simulator or Trainz. It's fun and relaxing gameplay.
If you need 100 hours to unlock all the characters (playing through lots of times was common for fighting games I recall) but you advertise you can buy them for $60, it puts a price on your time and takes the achievement of spending those hours getting better and puts it as "worth $60".
That's why some people make their own furniture, while other can buy it for $100.
Maybe it is A/B tested ? I doubt it though, I think it is an old feature.