Car infrastructure takes so much space that you can repurpose parts of it in place. Just turn one of the lanes into a tram line, make dedicated bus lanes. On huge parking lots you can just split parts of and build housing or more smaller stores there.

Of course there are limits to this, but cities are often grown historically over centuries and city planning usually takes place in such constraints rather than planning cities from scratch. Don't let the perfect be the enemie of the good.