Alex, thank you for your comments. We will go the VSCode way : it seems to work quite well, the debug mode seems quite reliable and we are not "tied" to Eclipse. And I also like Josh's vision of bringing FlexJS as "1st class citizen" into VSCode ecosystem.
Also, I would like to share my views with you regarding the future of FlashBuilder, of the "dual-output" philosophy and also regarding the workflow you suggest (rigid and less-rigid environments). I do not want to ignite an opinion thread ("This is good" vs "this is bad", "you are right" vs "you are wrong" etc....). Should I send a message directly to you or do you think that an "opinion-based" thread has its place in this mailing list ?? Many thanks again Nicolas (for my information : do we have any idea of the adoption rate of VSCode among Flex/AS3 developers ?)