That's a yes from me. There aren't many '50s cars I'm interested in but that doesn't matter. More variety, more choices, more happy players.
It also doesn't matter to me that they'd be slower than modern cars. That's true for every car in the game more than 20 years old. A Skyline R32 won't keep up with an R35 GTR. A McLaren F1 couldn't live with a Veyron, which won't keep up with a Chiron. A modern Ford Focus RS will walk away from an S13 200SX.
You can close some of those gaps by tuning the old car and keeping the new one standard, but then we step further back and we have Ferrari 308s and 512s and Maserati Meraks and a whole bunch of other stuff that would need a lot of work to keep up with a modern hot hatch.
Regardless, I don't see the point in racing a 1950s car against a 2020s one. All of the races available have parameters, regulations, so cars from the 40s, 50s and maybe 60s would likely have a series allowing them to race eachother.
I had no interest in the Porsche 356 Speedster, but when I first put on the VR headset and took the driver's seat of one in one of the music rally things, I fell in love with its curved wings, dainty windscreen, and beautifully modelled dash. I loved that the top of the door was visible on my left and tried to peer over it down at the road (but was foiled when I headbutted the wall next to my seat
)
It's not a car I've ever had any interest in through looking at pictures of it or reading its specification, but in VR it's a lovely place to be. Doesn't matter how slow it is. Cars from that era had so much individuality, it gives them each a different character before you even get going in it. The body coloured dashboards, the slim pillars, the tiny dimensions of the European cars. The even smaller stuff coming out of Japan. The huge, long bonnets and massive tailfins of the US cars and their bench seats and cavernous interiors and laid back, rumbling V8s...
The modern cars, in comparison, with their pillbox windscreens, huge A-pillars and digital dashboards, in my opinion, can't compete as a nice virtual place to sit.
If GT added more '50s cars to the game, I'd discover more gems that I'd previously never have given a second glance. To me, that's what makes Gran Turismo special. Every car is interesting for what it is, not for what it can beat. I can drive a bunch of cars, all by myself, around a track for hours, not care what lap times I'm setting, just enjoying that each car looks and feels so different. I can discover versions of cars that I never knew existed, or "drive" cars that were on my bedroom wall 30 - 40 years ago.
To me, Gran Turismo is not a car racing game, it's a car experiencing game, and VR has just made that experience so much more immersive. The more cars it has in it, if the quality of the modelling remains high, the better the game will get. They don't seem to have a history of massively growing the car list unfortunately, but with the VR world now, I really hope they do. I'd pay to support new model development, because where else can I experience dream cars like this? I'm never going to get to sit in a real McLaren F1, never mind drive one. With the second hand market the way it is, that's becoming true of a lot more classic cars. This is as close as I'll ever get to most of them so I'd love it, but not expecting it, if PD do expand the list. A lot.
They could also go with more '50s racers which opens up a whole world of quality, from the Maserati 250F to the BRM V16 to the Maserati birdcage to the original Ferrari Testa Rossa. As Nick Mason and Mark Hales showed in their book Into the Red, road machinery, at least in the 1990s, still hadn't caught up with the pace of 1950s racers. They were still fast cars, and remain so now.