DF will not go mainstream as it is a massive niche title. Graphics will not change this.
I disagree, there are quite a lot of df clones (rimworld, gnomoria and banished to name a few). The base gameplay of df is far superior to all of these games. The only reason why (some) of them are more popular is the graphics, which makes the game far more appealing to most gamers.
I have put 900+ hours into df; if there were no tilesets, I dont think I could play df and I am sure a lot of other df players don't find df as enjoyable without a tileset.
It all depends on how the game is viewed by the players, which is where the art of marketing is the answer.
First of all, how the game is presented in the media - the most complex game ever, most hardcore game ever, not for casuals, losing is fun, worst graphics ever. This is the general image of the game. Graphics and interface certainly help to keep it a lot. It certainly does not help the game get more players.
If, on the other hand, this image was replaced with - the game in which you can be whoever you want to be, where you can build epic fortresses, dozens or hundreds of games combine in one, the ultimate fantasy RPG, and if the game's numerous features possibilities were clearly stated in a way that the players can know at a glance how much they can do, then it will certainly open it for many players - as I stated before:
- You want to build a city/castle/fortress? You can! In the most advanced system yet to be developed. Replacing the word "complex" with "advanced" would certainly help.
- You want to play roguelike? DF is the best roguelike of all, ultimate roguelike, if you will. It has the most realistic world, richest gameplay, the most possibilities, countless enemies and so on, so on.
- You want to be a farmer - we've got the most realistic, yet very approachable, farming system. Grow numerous crops (a number would work best, like 100+ or something), fertilize, sow, reap, and sell your goods in the nearest city. Invest the money to furtherly expand your farm. Feed the fantasy world!
- You want to be the tavern owner? There are quite a few games and mods that allow you to do that, sure, but in here you can have the best system - produce and sell drinks, farm and cook delicious food, listen and spread gossips, treat the wounded, fight the bandits and the creatures of the night, entertain your guests, choose the music, decorations, and many more!
- You want to be an artist? Wander the massive world, tell enchanting stories, learn the world's lore and compose poems and songs about the bravest warriors and greatest victories! Or achieve ones yourself and the compose epic songs about yourself and they will be sang for generations!
- You want to be a thief? (In the future) We've got the most advanced system for you! You can be a thief, join or form a guild, steal thousands of different objects in thousands of cities and sell them to numerous fences or unsuspecting customers.
- You want to be a bandit/a thug/a marauder? Well, you can. Rob people, prey on small villages, loot and murder the innocent. This is a game for you!
- You want to be a merchant? Well, you know how this works by now.
We need to create a positive image of the game. That it is complex, yet approachable now with the new graphics and additional programs. In the future it will be highly customisable with different fantasy levels, brag about it! You can roleplay more professions than in any other game in a more advanced and rich world than anywhere else.
But ultimately I believe that you can't do it with ASCII graphics, and that is why games either abandon it (like Cataclysm) or simply begin with tilesets - RimWorld, Prison Architect, Stardea Valley and many more. DF has competition now, and those games are much better suited for new players - approachable, with clear graphics, and most of all - they know how to advertise themselves and to create a certain image that attracts new players.