0 AD has been in open source development since 2009.
It has been in alpha for 14 years, and currently there is no realistic timeline or roadmap for when it will leave alpha. In fact, I'd lay better than even odds that the project never will leave 'alpha'. The reason: nobody seems to have more than a very vague idea of what that would entail, and I can't imagine this group ever coming to a consensus about it when we can't even agree on basic gameplay requirements.
This is not necessarily as bad as it sounds. Actually I think it is admirable to acknowledge that this is not finished software. It's true that "the 'Alpha' label is scaring off new users from trying the game," but for many of them this is fortuitous because it correctly calibrates their expectations. No one currently downloading the installer is expecting a complete experience and coming away feeling deceived about the current state of the game.
The problem though is that the alpha label implies a development process that doesn't actually exist. It makes it seem like if you just wait a few more years the project will be more complete, and that is not true. It is more like the open source equivalent of software as a service, the product is not feature-complete and maybe never will be, but it is still a whole and cohesive product made available in its current state for those who can take it on its own terms. For everyone who can live with that, play away. For the rest, look elsewhere for your free ancient warfare RTS fix. However, that is not what the alpha label implies.
Personally I think the most honest way to thread this needle would be to drop the use of alphas for all future updates, but also rebrand 0AD as a whole. It is not a game: that would require campaigns, and better multiplayer support, and performance improvements that may never arrive. Rather, 0AD is "an ancient warfare RTS sandbox".
And if it ever does evolve into a game, that just means it's time for a new title. 0AD: Empires Ascendant perhaps.