They do guess at times, and are rarely certain on a close play. They are great at what they do, but we have access to something better through existing technology in every major league stadium.
They should give JB Shuck some kind of award for being the only guy who ****ed up enough so that Hank Conger could throw him out stealing.
Not every play, but borderline ones are the ones they're basically trying to convince themselves they're seeing something they're not. Bang bang plays at bases that are proven wrong on replay are largely educated guesses. Or worse, they thought they saw something else, and the human error takes over (and in the cases of some older umps, with older brains, older eyes and older reflexes... It's a problem). There's enough technology out there to start phasing out the human element of the game that is fraught with error.
On those, the only guy that truly knows what's correct is the baseman. Not even a replay can tell you that. He feels when the runner's foot hits the bag vs when the ball hits his glove.
At this point, u keep them on the bases because it's faster. Just like why tennis keeps line judges. But replay largely picks up nearly every one of their errors as is... And if they don't get better, games will simply take longer and longer.
You underestimate replay... It's getting more and more definitive (either that, or umps are missing more obvious calls that require reviews to fix). Technology is definitely moving faster than the human brain/eye reflexes are evolving. Ball/base proximity sensors can't be all that far behind.
