I will be honest here: if the PEP 508 format is chosen for PEP 621, Poetry will not be an early adopter of it. I would consider it a regression (both from a user experience and programmatic standpoints) compared to what Poetry provides today and it would drag its development and planned features behind, so there would be no incentives for us to adopt it in this form.
The user base of Poetry is growing each day and having to tell people that they should specify their dependencies the old way from now on would be a major step back for the project and would do more harm than good.