The markdown format of the legacy editor has always been my primary reason for using Trello, so to see a new editor be added that blasphemes it with a vastly different system has me worried for future use. If there is an intention to no longer allow switching off to the Legacy editor, it would pretty much immediately result in myself preferring to find a different service instead of waste time learning something over the format I've used for almost 10 years.
I think it would be ideal if there can be some confirmation of rather or not a toggle will be kept permanently to switch between the two. It's already in a pretty out-of-the-way location to swap back to markdown, and given that it's in the Labs section it gives the impression it will be removed eventually.
Would it not be better to just have both systems as an option so people like myself who use markdown do not feel compelled to find a different service?