Whenever people criticize an idea, challenge them to propose an alternative. It's easy to be unhappy, but if you can't offer a better solution, then you're just complaining.
In situations where you're trying to get people with diverse interests to agree on how to move forward, invite them to share full-fledged solutions to the problem before having an in-person meeting. Highlight that the final outcome will be a synthesis of these submitted proposals. This increases the probability that everyone understands the key trade-offs and that everyone has an opportunity to be heard.
By giving everyone time and space to think, it's more likely that you'll have a productive and level-headed discussion where people are open to compromise. Meetings shouldn't be about thinking of new ideas on the spot, rather they should be about sharing how everyone's proposals are being synthesized.