Could gliders be a clean, practical alternative to other forms of transport?
There's one thing that will forever prevent an unpowered aircraft from entering commercial passenger service: the inability to make a second attempt at a landing.
Airliners, with all their instrumentation, very experienced pilots, and such don't make go-arounds frequently -- but especially during weather, they do make go-arounds.  A passenger sailplane absolutely can't make a go-around, so if it gets crossed up due to a crosswind gust low and slow, instead of advancing throttle and climbing out, all the pilot(s) can do is pick where and how hard to crash.
This complete lack of second chances, as much as any of the other objections raised in other answers, will ensure that unpowered aircraft will never see commercial service.  It can also lead to &quot;landing out&quot; where you wind up having to retrieve your passenger glider from a farmer's field, freeway, or worse, along with its payload and crew.
The other big issue is how to launch these immense sailplanes -- with a common sport glider or competition sailplane, you're towed into the air by either another airplane, a winch, or occasionally a car or truck.  The winch that could launch something the size of a 737 high enough to fly off in search of its first lift would be so big and expensive it would severely limit the number of airports that could service such craft.  The airplane that could do it would be at least as big as that 737.