Creating tilesets can be extremely time consuming, Especially if you want the tileset to actually be good.
It isn't that Zone, Or any variation of Graal would likely be bothered creating a whole new better tileset and it also would not be difficult at all to implement given how Graal's tilesets work, Hell, I made tilesets for some games I made myself numerous times and it only takes roughly 5 or so days, Depending on the detail and what it is.However there is just a lack of reasoning to do so I'd imagine, Changing the graphics or improving them, Will not make a game more lively and make more people play it. Especially when graphics themselves don't make the game, And are often very opinionated on what is considered good when taking peoples opinion person by person. Zone should likely focus on other, More vital things than just graphic work.
People oftenly start and play games because they are interesting in nature, Not because they are necessarily beautiful graphic wise, Unless the game's content is built upon the graphics which is usually very niche and rare cases, And obviously don't apply to Zone, Or any Graal game. So it wouldn't make Zone "Not dead".