Some people (you?) get an enormous amount done without any kind of timeline or plan.
If the timeline was up to other people there would never even be a timeline, and nothing would get done.*
The purpose of a timeline is to help us do things together in the right way and at as-close-to-the-right-time-as-possible.
Imposing your (faster) timeline will probably speed things up – but it will also mean that it’s your timeline. The blame for every moment of pressure and stress, every rushed mistake, every feeling of interpersonal friction and discomfort – even every totally-unforseeable-and-unavoidable complication** – is yours now too.
This can be fine: imposing your timeline will get things done, and people may end up thanking you for it. It is sometimes (often?) necessary to work like this if you’re the boss – your mission may depend on it.
But a negotiated timeline is often better – especially when working with volunteers. It’s likely that your team will experience exactly the same levels of pressure and stress, rush and error, friction and unforseable complications – but they chose this timeline, and as a result these problems now belong to the team, and the blame for the unforseen goes to the universe, and not to you.
*It’s better to have fewer of the second kind of people… but planning and productivity are in some ways relative, so it will probably always seem as if you have lots of them.
**And the late night drive to the copyshop to fix the complication.