Who has the right to be glib here - Marty McFly or Emmett Brown?

Right. So. I was watching Back to the Future III recently. Now, call me petty, and what with the plethora of other time-travel paradoxes just kind of overlooked in said movie you'd be justified in so doing - but I have a quibble Mr Zemeckis!! On two separate occasions Marty is jibed glibly with not thinking 4th dimensionally by the Doc - on the first he's afraid of smacking into a wall that didn't exist at the point in time where he's going, and the second time of running off of a bridge not yet constructed when travelling 100 years into the future. Now, as with most people, my tendency was, once again, to sit back and laugh smugly at the hapless Mr McFly and think to myself "ah, Marty.... what a dufus". But is this really fair ladies and gentlemen? If I were to say to you "This delorean is a time machine - it will take you back in time only when you get to 88 miles per hour". Supposing you then travel back two months. Is there not a case to be made that if you arrive at the same point in space, but at a different time you'll end up in the vacuum of space? Is it not at least a little surprising that the delorean seamlessly ends up on the future (completed) version of the bridge at which it reaches 88 mph in 1885? Hmm? I think Marty raises a reasonable concern. Pointless post over and out.