Not necessarily. If it's a non-stop, you can leave the luggage downstairs, no issue of anyone taking off with it. If it has just one or two stops, you can "visit" your luggage for a few minutes at each stop and then head back up.

For my annual trip to France in July, even when the luggage is in the same car, I always check on it at each stop. I also try to choose a non-stop train if possible, so I can rest if I want.

That's funny, I usually prefer the downstairs because it feels larger and more open. The upstairs ceiling is tightly sloped and you can't fit luggage overhead. And there are luggage racks closer to the seats on the bottom level. But I take about 100 TGV trips per year so I see things differently than a tourist. And I am always in First because I have a half-price coupon.

To each his own. I can barely stand up straight on the lower level and find it to be airless and always too hot. I've only done the lower level in 2nd class, though, so maybe that makes a difference on the air issue.