There's always dreams that the #1 pick will become a Hall of Famer or multiple year all-star, but there is generally so much uncertainty in player development, that there is usually no clear path defined for that. The Phillies last foray into having the #1 pick was when they selected Pat Burrell in 1998. That worked out fairly well; only a few players have arguably had better careers from that first round (CC Sabathia) or flashed similar skills in a short time frame (Mark Mulder, JD Drew, Brad Lidge, Carlos Pena).
Which is to say, that the perceived success of this pick is not only predicated on the success of the Moniak as a player when he makes the big leagues, but also upon his relative performance to the other picks considered for his spot. We are a relativistic society after all. Here a couple #1 picks from the past that had different circumstances around their selection as depicted by 2009 obak auto cards and a nice ice parallel from 2012 Bowman that I pulled from packs.
Moniak is not really a comparison since he's at a different stage of development.
Moniak also doesn't compare to Goodwin, though they both started from up the middle defensive positions. Solid defense and a good bat from one of those positions makes for a valuable contributor to a team.
If Moniak could get to a level where he approaches Harper's down years (118 to 133 OPS+) for many years, that would be a win of a draft pick. He's seems to be a multi-tool centerfielder with good contact skills at the outset, so there is hope. It looks like we will see the progression by 2019. That's a lot of time to dream.