^That was my thought as well. My brain is cluttered, so I don't remember which options they liked from
this document in each W7th segment. To Silophant's point, there are two major cost buckets of future changes- extending the platforms for longer trains, and converting mixed traffic lanes to dedicated lanes. The latter isn't that expensive assuming all signal work is done at the outset, just some barriers where need and better signage? The challenge is political. Except for the 3-lane concept, which would be a lot of new trackwork and moving curbs/stations.
Given the layouts of most of the side-running concepts, I think it's not crazy to say a platform could be extended along the sidewalk with little disruption or cost (tracks stay in the same place). Most side-platforms have an additional sidewalk outside the platform, so you're only running up against the political challenge of losing more parking - of course this assumes the stations are located such that a longer platform would be in a place where closing a side-street isn't a problem, which are frequent thanks to W 7th being at a diagonal.
This is why I like the center-running design better, even if it's shared use to begin with. You can extend platforms down the middle of the street with the curb staying in the same spot. This also allows all side-street corners to keep their access. Initial design matters, since building a longer platform means a new track approach unless you curve them out in the initial design. Side running, even with dedicated lanes, will get held up by turning or parking cars, center-running (when dedicated) won't. The only downside is we only get parking on at most one side, and we lose left turn lanes. So, we all know which one they're gonna go with.