Interesting thread!
It's definitely more about variety than sheer count for me. 140 could be a perfectly good number in GTS if it was varied, so far it doesn't really appear to be. It's LMP1s, GT3s, GT4s, road cars and fantasy knock off versions of those real classes. Oh and rally versions of those road cars. All of them are from the last 7 years. I'm actually surprised more people aren't making such a fuss of that, I know we don't have the full list yet and with the delay there is scope for change, but considering the past car lists it seems strange more people aren't bothered by the tiny range of cars in GTS.
I am bothered by this - sufficiently so that I might skip GTS altogether and so might most of the friends that I race with online.
My hobby is hosting online championships which are inspired by historic events or the careers of past drivers - the sort of stuff which could easily be in a game, but hasn't been so far in GT. We use historic race and road cars in a realistic way rather than the random mix approach of GT5 and 6 which pits cars of different era's together. Currently we are running a tribute to Dave MacDonald (Corvette and Cobra Legend) using the Art Morrison Corvette (C1), 1963 Corvette Race Car (C2), Shelby Cobra 427 and the XJ13 Jaguar painted to replicate a Cooper Ford ("King Cobra") or Cooper Chevrolet. Past championships which have been very popular have used the Classic Mini mixed with substitute cars to recreate 1960's saloon racing.
I bought Project Cars Game of The Year Edition with the intention of switching from GT6 in Spring 2017, but have been very disappointed with the car list - there is classic or historic content, but it is randomly spread throughout the list in such a way that it can only be used for single make racing. It is as if the person selecting the cars had no knowledge of motor racing history. You get a classic Aston Martin sports car, but there is nothing to race it against and this story is repeated throughout the list.
It is perfectly possible to put together a list of 125 or 137 cars and create realistic and interesting series for both modern and historic race and road cars.
Sadly Assetto Corsa (so far) also looks as if it's historic content has the same random approach adopted by Project Cars with the result that the historic cars will only be of any use in single make championships.
Kazunori said that classic cars were selected based on their historic significance, but the implementation of this to date has been poor and very random - suggesting a lack of strategy, vision and knowledge - however us classic fans were able to put together some interesting series on GT5 and 6 despite this, by using substitute cars / replicas because the car list was sufficiently large.
There is a potential positive from the current GTS car list - the presence of FIA championships has forced PD to think about a structure (classes) into which to fit the car content. If the structure is extended and applied to historic and classic cars when they are added to the game we might be able to hold realistic races with online friends (or via an event creator). PD would need to swot up on their motor racing history, identify the iconic cars from a range of era's and motor sport categories and then add an appropriate number of competing cars. For example: the motorsport icons of the 1960's would include the Ford GT40 (Sports) the classic Mini (Saloon) the E Type Jaguar (GT). In many cases the competing cars already exist within GT6 so just need cleaning up for GTS. If as well as identifying the iconic cars on which to build a car list strategy PD also identified those cars which have special historic significance because they crossed motorsport boundaries PD could then have a truly versatile list. For example the 1963 Ford Galaxie raced in NASCAR and virtually the same Holman & Moody prepared cars also raced in the BSCC, the ETCC, Australia and South Africa (the Springbok Series). Another example - The Lola T70 won the inaugural Can Am championship, but also competed throughout Europe including at Le Mans. This sort of approach to the car list yields cars which can be used in multiple series so increases variety while reducing the amount of modelling required.
I find it unbelievable that all those premium car assets from GT5 and 6, which we were told were future-proofed, are seemingly going to waste in GTS. There is the potential for PD to make a truly great game - I sincerely hope they do.