What are you defining as "complete the game" though?
Good question this: I go based on the ending movie. This is different from getting a platinum trophy (I'll touch base on that again I suspect) but once you get the "ending movie" for a game you have completed it.
That doesn't mean there are not things you can still do within the game, however. Side quests, secret unlocks, DLC and the like litter the videogame industry. Gran Turismo is no different in this aspect.
Getting to the finale doesn't require them, but it leaves a lot of game that hasn't been completed and doesn't take especially long. You might only "require" a seven-figure car - a Super Formula - to complete all of the Menu Books and have your "Event Directory" at 100%, but a few others into seven figures will make your life easier in events like Neo-Classical.
Both things you mentioned I believe underscore my points. Lots of fun to be had, but it's not required to complete the single player campaign. Menu books, Extra menus, Music Rally . . . these are all things allowing a player to continue to have fun after they "beat" single player mode.
And easier options certainly do exist! Getting cars that make beating the Neo-Classical events to make things easier is a choice, but not a requirement.
You can take a Red Bull to the Sunday Cup, but that's not required.
If you want to add the Extra Menu Books to that you're going to need more, as there's several Legends Cars needed to complete these (and a few Invitation cars, which is its own discussion) - some into eight figures. Should you just class the Platinum trophy as completion, you need to own at least four cars in excess of 3m credits (three of which are far in excess of it) to get that.
True.
Of course the elephant in the room is the Car Collection section of your Garage, which displays (almost) all of the cars in the game shaded out until you have collected an example of it and your progress towards that in the top-right. If you're defining "complete" as "nothing else left to do that will change how any game completion metric is displayed" (which seems like a complete definition of complete), this is the biggie because it absolutely does require all the cars.
Excellent point and I think this is the crux of the argument here.
Some folks don't consider a game "beaten" or "completed" until they have unlocked every single feature, beaten every single challenge, collected every single weapon/part/crown/mission/quest/thing in the game and run through all the optional choices (Dragon Age: Inquisition I'm looking at you here!)
Still others hold the mark of getting a platinum trophy.
The developers have a goal which provides the ending/credits for a game. I use that as the end because speaking as a novice author, I know when the story actually ends. I could put up addenda, publish short stories in the world I created but it doesn't change the fact my story had a beginning, middle and end.
I give the same deference to the developers of Gran Turismo 7.
Mind you, only a handful of extremely high value (and Invitation) cars are required to be purchased to complete this last one, as the majority can be won through tickets. Which takes a while 
INORITE???
what if you want one of these ? for the pleasure to drive it ? or because it is locked behind invitation and you don't know if you'll ever get another chance to obtain it ?
I'm in this boat and I love just driving cars in V.R. But it doesn't change what I mentioned above. It's a choice, not a requirement.
and I am tired of the argument of "completing the game" it is non-sensical with GT7, it is not a game you "complete" but rather you play it until the next one comes up (it does not prevent you form achieving certain goals you can choose, but there is always things to do once those goals are reached).
If I beat "story mode" in a fighting game, I can still play vs. the computer or vs. friends or the like . . . continuing to unlock things as I go along. But I still beat story mode. The developers agree I've completed the single player part of the game and as a result I have the credits scrolling down my screen.
I can still continue to do things in the game afterwards and I've beaten the game.
These two items are not mutually exclusive.
and guess what, for some users (not me), "completing" the game is getting all the cars (and that's also how it has been pushed/advertised)
can you "complete" the game without getting all trophies ? oh no "completing" the game is not that either ?
some will have an idea of what "completing" the game is, and many won't have this need of "completing" the game anyway
Sounds like what I said above.
and could you please stop shifting the goalpost ? You said "The economy is only an issue if you want to collect every single thing..." and now it is "the economy would be an issue if half the cars costed 3M"
I don't think I was moving the goalposts, but let's look at the possibility for a moment.
The economy is really only an issue if you want "All the Things" in the game . . . specifically the expensive ones, and you don't want to take the time/patience to earn the credits required to get all these items in a month/3 months without microtransactions. I haven't shifted from that.
The fact is most cars
do not cost 3M. If they had -
and they do not - I may be questioning the economy. But the truth is very few cars cost that amount compared to the whole.
That's not shifting the goalposts. That's simply stating the way things are and stating how my position may have been different if things were not the way they currently are.
So I think it's clear I'm not shifting goalposts here.
sorry for the log reply . . . but I'm using the multi-quote to keep my post count down.
;-)
