My point is this. Will 1D&D be backwards compatible with 5E, probably, But with lineages, class features, feats, spells, weapon properties etc changing, will it be practical to do so probably not. If I understand correctly as of the last playtest aren't feats no longer considered optional? Hypothetically, if I was starting to DM a game of 1D&D and had 6 players excluding myself and 1 of those players decided "eh I'm just gonna play a 5E PC", I'd probably tell them to take a hike unless they made a real convincing argument. I wouldn't want to pull double duty reading/referencing 2 sets of books. The playtests and the community's feedback is not the be all and end all, I am certain that yes they will definitely take it all into consideration but OTOH I can almost guarantee that the final playtest and actual releases will be slightly different. At some point the playtests and surveys will close but that does not mean the final design is complete. Of course, this is all just my opinion, and I could be glaringly wrong.