See below quote.
As for your last point, it keeps engagement levels higher. Yeah you can buy cars with credits sure, that's fine, the racing needs to be fun of course to make the credit earning enjoyable. But there's an excitement and a fun to be had in unlocking special cars through a reward system like the manufacturer affinity from previous games. Horizon does a similar sort of thing with the seasonal unlocks, plenty of other games have done it too. It makes game 'rewarding to play. Of course you have things like GT7 which go the other way and make it almost impossible to get certain cars no matter what you do and has a pathetic level of grinding required. FM4 is the sweet spot.
Maybe that's where the previously mentioned racer vs gamer thing comes in. Racers play for exciting races, gamers play for shiny unlocks I guess.
And bear in mind GT Sport was pitched as an online focused game. Didn't even launch with a single player campaign. Those videos if I remember, were mandatory to play online, so approx 20% of GTS players played it online according to those figures, an online focused racing game. A game with emphasis on single player, ergo, main Gran Turismo games, Forza Motorsport etc, will likely have an even stronger weighting to single players.
I don't necessarily think that GTS to FM7 is an equal comparison, given the implementation of multiplayer in the prior iterations. Before GTS, I would have definitely agreed that GT had a single player focus. Sure they had multiplayer in a few prior titles, but it didn't seem very well implemented or populated and GTS was the first time it felt like they were making much effort for it, interestingly going basically all-in on it. Comparatively, the Forza series had well implemented multiplayer from the very first title and it was a much more evenly split population judging by my own experiences within the game and in several other communities, even if that isn't the case around here. Note that I didn't say "even split" but rather "more evenly split" because yeah, it's not 50/50 but I really doubt it's below 20% on the Forza side of things.
It makes plenty of sense that a game with 6 prior titles with well implemented multiplayer would have a better established multiplayer playerbase to bring forward into a new title than a series that had a few entries with half-hearted multiplayer would.
I'd also dispute your claim thay multiplayer is what bought Forza Motorsport to existence. Microsoft and Xbox didn't have their own, proper Gran Turismo rival, Gran Turismo is a console seller and MS didn't have that. Sega GT 2002 filled that hole for a couple years but never hit the heights of Gran Turismo or latterly Forza. Whilst there's no doubt Forza was a big title for xbox live, again, Sega GT Online beat it out and it wasn't the first online racer. It was a direct response to GT3 and 4.
To be clear I wasn't saying it is what brought it into existence, rather that multiplayer implementation was one of the key things it used to steal players away from GT/Sony, along with the customization, and make it the success it is today. Yeah, it was primarily Microsoft's GT fighter, but the online clubs/racing/etc was a big deal back then, and its consistently solid implementation of those things are a big reason why a lot of people stuck with the Forza franchise over returning to GT... at least until GTS, which did steal some players back from Forza multiplayer community.
Keep in mind I'm not saying it is a primarily a multiplayer game either, just that I don't think its as insignificant as some people around here like to make it seem like it is, and it's enough of a factor that it should earn some right to compromise in game design areas, including making choices in regards to content and how it is delivered, rather than the "meh, it's a single player game, **** 'em" attitude that it often gets around here.
Now I'm not saying online isn't important, it is, the portion of dedicated online players might not be as large as the single players, but they're naturally more vocal. The tests were online focused because the game is undergoing a reinvention and a shift. Its easier to get that right with a single player campaign than it is with online. If the online systems don't work as intended or aren't fun, have issues etc, you've got a big problem, so it makes more sense to take a small dedicated segment of the online player base to test that. It certainly doesn't mean the game is going to be an online focused racing game. It means the online section needed real world, real player testing.
I know you aren't saying it isn't important, but there are some around here who blow it off in thinly veiled "well I don't care about multiplayer so no one else should" kind of comments and it can be hard to tell with some and the lack of tone in text.
I also didn't mean that the multiplayer beta tests were a sign it was multiplayer focused, rather that it was a sign that they don't see it as just some extra mode for a small fraction of the community to play.
Not sure I'd agree with it being easier to make single player though... I mean it should be in theory, but it given how bad career modes have been for a long time it doesn't appear to be the case. Especially with the big reliance on AI which seemingly hasn't improved much in the last 20 years. It's part of what will make this game's launch so interesting, it's desperately in need of both career and multiplayer updates, although the multiplayer issues are primarily rooted in the crashy culture (hard to fix, but easy to work around) and the shallow racing (which we already know they are trying to fix with tire compounds/etc). The deeper racing will also help the career of course, but in my opinion it needs even more than just pit stops to make its career interesting again... Then again maybe it's just me being too jaded to enjoy any of them at this point.