A hard deck within - say - 4 miles from the finish line is a simple alternative. Get below finish height and you're scored as a land-out.

This is indeed a simple and better alternative. It eliminates all this worry about pilots thermaling low to get over the edge, as well as last minute pull ups. (A pull up to get over the hard deck won't work, as you will run out of energy.)

We don't have it, because you see the uproar that a simple finish cylinder is causing. Mention the words "hard deck" -- even just a doughnut at 500 feet AGL, 4 miles around the airport -- and there will be RAS apoplexy. I brought it up once, and the rest of the RC pointed out wisely that if I wanted to go get tarred and feathered that was fine, but they weren't going to join me.

Don't do big pull ups at the finish!

Once, approaching Hobbs at about 70 knots -- I was on a pretty marginal glide to the cylinder -- about 1.1 miles out a glider rose up right in front of me. He had passed me below at high speed, somehow missed the glider above him (me). Staring at the airport on final glide is common. At the moment of the pull up, I couldn't see him -- I'm above -- and he couldn't see me -- now behind his tail.

When you do a big pull up, there is no way to see who is above and behind you!

John Cochrane