So, lost in a lot of the Russell chatter was the nugget that baseball prospectus intends to rate Alcantara #18 in their midseason rankings coming out today. Not to say that he isn't deserving but they had him at #83 prior to the season. Talent varies from season to season but generally when you start talking about top 15 prospects you're talking about players you build a team around usually. Historically the attrition rate of those type of players is 62.7%/39.6% success/superior for players ranked 1-10 on baseball america's top 100 and 59.3%/39.5% on 11-20. Granted we're mixing rating services but given the season Alcantara is having it would not be surprising to see them there on BA's midseason list. In this data, "success" is anything over 1.5 fWAR/season average while "superior" is anything over 2.5 fWAR/season average. Another interesting tidbit in that data is that if you look at success rates of finding "superior" pitchers outside the top 10 prospects you basically are talking about needing 2 top 30 pitchers to equate to one hitter in the top 20(39.5% superior rate for hitters in the top 20 and 18.7%/14.3 for 11-20 and 21-30 respectively). This lends credence to those who are suggesting Russell is worth every bit as much as two top 30 pitchers by himself.
The source of this is found here and I recommend it as a very interesting read.
http://www.royalsreview.com/2011/2/14/1992424/success-and-failure-rates-of-top-mlb-prospects
Additionally, if we extrapolate the data and look at the rest of the cubs prospects you're talking about roughly a 30% chance of "success" and a 17% chance of "superior" for position players and a 19/7% chance on pitchers. I would exclude Schwarber from that area for the moment. Kohl Stewart was the #4 pick last year and started this season in the mid-30's. So, given his similar draft pick rating and his start thus far it's likely Schwarber will be around there with the potential to be in the top 20 soon if he continues to hit.
If we bring all this together and assume Schwarber does end up in the top 20 at some point realistically you're talking about 3 of Baez, Russell, Bryant, Alcantara and Scwharber averaging 1.5+ WAR/season as well as 2 of that 3 becoming 2.5+ WAR/season types. As for the rest of Soler, Almora, and possibly McKinney, Vogelbach, Eloy Jimenez, and Torres you're likely talking about 1-2 of them being 1.5+ WAR/season types and 0-1 of those becoming a 2.5+ WAR/season type unless they rapidly advance up the top 100 a la Alcantara. As for pitching, with Edwards, Johnson and possibly Tseng in consideration for top 100 you're likely talking about 0-1 of them becoming a 1.5+ WAR/season type. As such, if we assume Castillo, Rizzo and Castro as established players who are likely to stay you're adding 4 names out of the cubs top 15ish prospects to the line up with maybe 1 pitcher to a staff of likely Wood and Arrieta going forward.
There's going to be additional talent coming outside of the 15+ prospects the cubs have as well though it tends to be way more volatile. Either way, I think it's quite likely they will be able to field the majority of a MLB lineup with their current prospects even despite those who will suggest high prospect bust rates. And unsurprisingly, they are likely going to have to add pitching some other way. However, having spent a lot of lessor draft picks on pitching they are likely to be able to fill out the back of their rotation with players it's mainly the top of the rotation in question. This again, is rather unsurprising. However, if that is how things play out they are going to have a relatively cheap line up which should open up a lot of financial assets for pitching. Even in 2019, you're only talking ~$29 mil between Rizzo, Castro and Soler though things likely are to get a bit tougher after that unless they finally manage to secure a huge TV deal as I believe 2019 is when the CSN portion of their tv contract is up.