No to an option to have all cars available in Arcade Modes (Arcade Race, Time Trial, Drift Trial and Custom Race) and Two-Player Battles. A handful of cars is the way to go, and not all cars is a kind of lame idea.
You talked about Forza, Forza is a FREE ROAM game a Gran Turismo is a RACING game, Gran Turismo has race tracks where Forza you drive on roads.
Motorsport, not Horizon, and plenty of very, very po-faced and serious sim titles (the majority far more racing focused than GT is) allow you to test drive cars, hell most of them give you access to all the cars from the very start, because they focus only on the racing, with zero car collecting involved. RaceRoom goes even further and allows you to test drive DLC before you buy it!
You can argue which one you prefer until the cows come home, but claiming it's 'correct because GT is a racing title' is demonstrable bollocks.
This idea of having test drive cars in a Gran Turismo before you buy is really an absolutely very a bad idea, because GT games have race tracks and that means you race on them. If you really want to test drive a car do what real life people do, test drive the car on road before you buy.
You were wrong about this yesterday and you are still wrong about it today.
In reality the vast majority of activity that takes place on a track is not racing, it's test days, driver training, experience days, track days, product launch training, vehicle development, etc. The vast majority of circuits would not survive if they had to solely rely on race events to fund them.
Silverstone from example is holding less than 20 race events this year, and over 100 experience sessions alone this year, so even without track days, driver training, test days, product launch and development sessions, that's more than five times the amount of race events. That's for one of the busiest circuits in the UK, add in the rest and its likely to see a ratio of well over 10:1 non-race to race events, circuits try and book activity every single day if they can, as it maximises revenue and viability.
Also good luck test driving most race cars on the road, that will not end well at all.
The degree of ignorance that you are attempting to use as if it were a source of authority here is staggering.