Adding slower cars just for the sake of it is not really a good move. Specially since they are trying to please very different kinds of players with very different tastes in cars.
Close racing can happen with any type of car, they don't need to be slow, and it relies more on the drivers than the cars.
At this point, it's safe to say that we have a pretty solid list of slower cars. They make up for some 7% of the car list, and I'm only counting with cars below the DS3 Racing and Abarth 500 as "slow cars". And with engine swaps being a thing, they will get a bit more usage from the rest of the players (that otherwise wouldn't touch them).
.....but his point isn't adding slower cars "just for the sake" of adding slower cars. There is legitimate reasoning behind it of which I happen to agree with. .....besides some of my own points of the view on the topic.
For one, slower cars are much more forgiving to drive and allow much more time for players to assess situations, plan and react. Simple mistakes don't necessarily turn into costly consequences like dropping a tire off the edge of the track or being nudged traveling 150MPH down a straight. They are easier to recover and keep racing.
There's also something to said about these more common car types that tend to speak to the everyday driver. How many times have we heard people reminiscing about cars they've owned and wished they were represented in the game. .....many, relatively common cars but people still form connections with them.
To your point, the fact that we are now able to customize these vehicles again brings it to a whole new level. Players are able to grow and appreciate their efforts by upgrading their cars. Maybe a dream they always had in real life but never did / could.
Don't get me wrong. Driving exotic & race cars is great. .....GT3 happens to be my favorite class but I also wouldn't mind a wider selection of more "everyday" cars that I can own and modify.
Being a 2010 Dodge Challenger R/T owner, I loved having the '08 represented in GT5/6. I was hoping the modern Challenger would make a comeback personally in GT7. .....as well as a 1969 Charger given I'm a fan and owner of that car as well. .....and maybe the 02/03 "bugeye" WRX because again, I have a personal connection with one I owned over 8 years.
I think these slower cars are simply more accessible to most and (I think) most will feel more of a personal connection with these that they interact with in their daily lives through ownership, a realistic desire to purchase and so on.