In reply to Anh Tran.

@Anh I've been building a theme to test out creating it for both classic and Gutenberg. I can already make the claim that developing for both is definitely adds a lot of extra time and workload.

For example, and even though I am not creating custom blocks, I am styling some of the core blocks in Gutenberg, such as the Pull Quotes. Messy, Messy coding. You need one stylesheet for the classic editor which is basic CSS like the theme style.css has. then another for the Gutenberg editor. For the pull quotes block, the fun begins when you realize you cannot simply put the basic CSS styles into it because only part of it will work. You also have to add extra CSS for the editable areas of the blocks which means now I have to style the editor's own containers. Example:

/* Pull Quotes */
.editor-block-list__layout .editor-block-list__block[data-align=left] .editor-block-list__block-edit

Fun times! So yes, development will become a burden.