Of course is money related, this isn't cheap to make. So you have to take decisions with your business in mind otherwise you risk future content and updates in the medium-long run.
The only place makes sense to race against cars you don't own is in iracing style game. There you buy every piece of content. Iracing isn't risking much by producing and selling piece by piece. But the risk is higher when you make dlc packs.
In fact the more successful the DLC is the more my existing Assetto Corsa products lose value in terms of playing multiplayer because fewer and fewer servers will host content I own. That's simply cynical unfair nonsense.
There are several servers that run daily and are populated enough with just base content. 458 gt2, formula tatuus abarth, 458 and m3 gt2, bmw m3 e30, toyota gt86.
There was someone in the AC forum that suggested to demo the dlc cars before buying. Do you know what this means? It means that people will try them for a couple laps and instead say they want to keep their 5-10€. But when people pay money for something more likely they will put more use to what they purchased, and in different forms they thought initially. Demos often result in unnecessary lost sales. Specially when you already know how the cars in AC drive and look, because you already sampled the game when you bought it, so you are familiar with devs work.
You can't have only the dedicated and highly interested people in buying dlc packs. You also need players mildly interested in the content to purchase so that it compensates the costs and investment for future content-updates. Otherwise what will happen is the rfactor2, automobilista, raceroom effect. Only the highly interested people will buy and this translates in low amount of sales. Which is a disadvantage for those who actually purchased those sims, they often don't have enough people to race against and the company has less means to expand and improve the game, or even to plan a new one.
