prosfilaes
Adventurer
There is no way to make that statement with any objective certainty. Seriously, depending on whose study you read, being first to market has between a 60-70% correlation of a product’s success, across all markets. Being #1 is a powerful advantage.
This is dealing in counterfactuals, so there's no way to make any statement with any objective certainty. And we're not talking about a product's success; if D&D went away today, it would still be a massively successful product. We're talking continuing market dominance, so it's less than that. I think if you'll look at that 60-70%, you'll find products that were relatively good, and moved quickly towards being good as competitors poked at them. I think that 30-40% is filled with stuff like Empire of the Petal Throne, where, if it had been first, people would said "cool idea, but a little strange" and one of the competitors would have come out with C&S or Rolemaster and stolen the majority market share.
If I stuck a copy of the Threadbare RPG or Puppetland or My Life with Master in my bag and took it back in time to the year before D&D came out, and tried to sell it, I don't buy that back in 2018, the dominant RPG would be this quirky little thing; in fact, I wouldn't be surprised if it was still D&D and we were still having this exact same conversation, forgetting the real first to market. I wouldn't be surprised if another RPG came out before D&D in real life, and failed to make a flash in the pan.
Look at submarkets of RPGs; the first science fiction RPG is Metamorphosis Alpha, which never had an expansion or a new edition (or even reprinting?) for 20 years, and now it's around mainly because of nostalgia and the OSR movement. Starfaring was released a month later by Flying Buffalo; never heard of it until today. Space Patrol, Space Quest have largely fallen down the memory hole; Traveller was the first enduring science fiction RPG, but hardly the first science fiction RPG. Superhero RPGs are in the same boat: #1 is Superhero 2044, and people get geek points for having heard of it. #2 is Villains and Vigilantes, which stopped printing anything new between 1987 and 2010 (and the story is far too long to fit in the margins of this page). #3 is Supergame, and it's not until we get to #4 that we have an enduring superhero RPG in Champions.