Remove the releases and there will be a significant reduction in game shooting. This will mean a significant reduction in planting cover crops, in feeding and in predator control. I do not think that will be great for wildlife numbers, plus initially predator numbers will be relatively constant whilst prey availability (game birds) plummets. So there will be increased predation pressure on the species you're trying to conserve, plus on farmed species, and then the predator species will also start starving and their numbers will crash.
Some shoots may continue but there would need to be significantly more investment in habitat management and predator control whilst getting significantly smaller bags, which will likely put pegs financially further out of reach for many plus reduce the appeal of the big estates to the rich types who just want to kill a lot. In itself the latter is not a bad thing but with it goes their money and many rural jobs. I'd expect a significant shift in the type of shooting, initially becoming more expensive and exclusive, then the market for shooting leases crashing, and then potentially a resurgence in small scale walked up rough shooting done by a handful of individuals with the permission. This would take a few years and the number shooting game would likely decrease, making it harder to defend politically. In addition, the money generated, jobs, and conservation arguments would be decimated, at the same time we'd see a massive shift in wildlife populations, not necessarily for the better. Those who enjoy shooting pest species to protect livestock and crops may have a grand time of it though.