Well, frankly, after the top 5 or so teams, just having two premier players is going to get you ranked pretty high. Looking at Sickels lists, those prospects are not evenly distributed. Just looking at other teams with a top 20 guy, Seattle has 3 in the rankings (4, 75 and ?? for the injured pitcher) so do the Indians and Mets but the Cards look better than all 3. A's have one player (Russell at 15 but nothing after him), so do the Tigers. Rockies have 2. Royals have 4. I think Cards, with two premier guys in the top 20, should pretty easily slot into most top 10 rankings.
You bring up an interesting discussion topic, as Markos alluded to, is a premier player or two worth more than 3 or 4 top 100 players? Personally I'll take quantity over quality. The failure rate of prospects is so high, even of elite prospects, that I downgrade a system like the Cards.
I looked through all the teams top prospects the other day and IMO they break down into 4 groups.
Group 1 are the teams like the Twins with elite talent and depth. Every team has 6+ top 100ish prospects and a premier player. Astros, Cubs, Red Sox, Twins and the Pirates.
Group 2 doesn't quite have the depth but has 4+ top 100 prospects. They also have some very intriguing players that could quickly become top 100 prospects. The Mets, Dodgers, Royals, Rays and Marlins fall into this category (though to be fair the Mets have Syndergaard and d'Arnaud who could be considered premier).
Group 3 teams either have 3-4 top 100 prospects, one might be elite, or 2-3 top 100 prospects but 2 of them are elite. This is the biggest group and includes the Indians, D-Backs, Rockies, Cards, Mariners, Rangers, Phillies, Reds, Blue Jays and the Nationals. The Cardinals could fit anywhere from 11-22. I'd put them somewhere in the middle because I don't think they have quite as much depth as some of the other systems.
Group 4 teams are just B. A. D. Let's say it all together, BAD. A's, Padres, Giants, Tigers, Braves, White Sox, Brewers and Angels.
I think you're selling the Mets system quite a bit short. They have 2 premier prospects in Syndergaard and d'Arnaud and a very nice pitcher in Montero. In addition Wilmer Flores, Steven Matz, Kevin Plawecki and Cesar Puello were all listed in the "Honorable Mention" section of Sickels top 75 list. They also have Dominic Smith who was their 1st round pick in 2013 and was considered the top prep bat of the draft. Dilson Herrera is an intriguing 2B prospect that has the tools to be ranked in the future.
The Royals have a much better system than the Cardinals as well. In addition to the 4 he mentions they also have Hunter Dozier and Sean Manaea from this years draft. Bubba Starling, if he can ever put his tools together, and Jorge Bonifacio are both intriguing. John Lamb has had injury problems but was a very good pitcher a few years ago.
I'd take the Mariner's Walker, Hultzen, Paxton, Peterson foursome over the Cardinals Taveras, Martinez, Wong, ??? (assuming Hultzen's injury isn't career ending). Though part of that is because I value pitchers much more than OF's and 2B.
The Indians I'll give you, though I aboslutely LOVE Lindor. I'd trade Sano or Buxton for Lindor. Boom. Done.