It would be near impossible to write/update the rule to allow runners to pop off the base briefly during the slide. Dave Cameron tried (https://www.fangraphs.com/blogs/dear-mlb-please-fix-the-slide-replay/), and the best he could do was some sort of 'if you're still over the base, you're still safe' rule, which seems like even harder to enforce, barring MLB installing cameras directly over every base. Flat bases I think fall apart when, as mentioned above, you think of how often first basemen are pushing off the edge of the bag to receive the ball. It would fundamentally alter the stretch they do to make the play, and I would guess greatly increase the amount of feet/ankles stepped on by the runner. Ultimately, I think runners are just going to have to adjust the way that they slide. We've already seen it on head first slides, with an increased emphasis on holding the bag or bringing a leg up to maintain contact. In the play against Washington, maybe it's just a matter of, if he wasn't so far off the bag, he wouldn't have had to slide back with the velocity he did, and wouldn't have popped off for those few hundredths of a second.