I think it very much depends on the individual. While I have not been to the park in France, I have taken several solo trips to Disney parks, and find that if I'm feeling a moment of loneliness, it does not last too long because there's so much to do, I can redirect my energy toward the next interesting activity (planned or unplanned).
I haven't tried NYE solo, so I can't tell you how I'd react, but you might have a better sense. A holiday is probably not the best time to take your first solo visit, if you haven't tried it yet -- but that's just my opinion.
That said, while I wouldn't plan my trip around NYE, if I already was going to be near Paris by myself during NYE and the only alternative was to have a quiet night at home or in my hotel, I would probably rather go out to a place where there are other people, even if just to people watch.