Creating a modern full-fledged "game of your dreams", even if not AAA, can take years if not decades when you're going alone. People join development studios so that they actually have the chance of finishing a project in a reasonable timeframe and so that they can focus on one or two disciplines.
But lets face it - in a company- there are two or three people doing the game-design, the others usually get not even a chance to contribute.
They are put into a silo, connected to a part of the pipeline and start crunching until the ilusion wears off.
Even if you get a chance to realize a idea- in a modern title, the fear of risk, sees to any new concept that it is marginalized.
So even if it takes years, i think by now, its far more realistic to produce a title as a indy dev or open source dev by now.
