While people like to bash the utopianism of TNG, the fact remains that from a business standpoint, it was the most successful post-TOS series. It was more successful than the dark and cynical DS9, for instance.
It was, but then it might be argued that this was because it was
first. There's that famous graph that gets posted here a lot, of overall ratings performance through the Trek series of the 80s, 90s and 00s, and I think the same steady decline would be seen regardless of the order in which you arranged the series. Voyager was quite deliberately a soft reboot of TNG in terms of tone and yet continued the decline just as DS9 and ENT did. I don't think we can attribute TNG's success purely to its attitude to the future.
Also, as much as DS9 wasn't a "success" like TNG was, I still find myself revisiting DS9 for more timely and pertinent themes and/or characters.
Personally, I feel TNG has dated the most noticeably of all the Trek series, even the original. While the original visually smacks of the sixties, I think its stories, characters and plots hold up better to the modern eye than a lot that TNG did. TNG just seems, to me, quite naive and preachy looking back on it. Some of Picard's speeches are cringeworthy in retrospect.
Don't (please don't) have all the main characters be on one side of every issue. There are hundreds of billions of beings in the Federation ... they all agree on major aspects of their own society?
They absolutely shouldn't, and there will be hot button topics in the 24th century just as there are now. But I don't think having a current social issue reflected almost verbatim in 24th century humans makes sense or works in the story. It would look as out of place as teenagers in 2016 New York being shown discussing the best ways of identifying witches.
By all means, they can have an issue they fall on both sides of, but it should be a new one, specific to the time. DS9 sort-of went there with genetic engineering, but didn't push it all the way or allow it to develop as an issue. In retrospect, that choice seems outdated, as its more likely to be a real world issue of the 22nd century, not the 24th, but at least it is a morally ambiguous concept with good arguments on both sides which doesn't require our characters to appear like bigoted jerks from a 21st century perspective.