Final Tally
As of September 26th, the team of former Cardinals and the current rendition of Cardinals has literally the exact same fWAR total. Both squads have 26.7 fWAR right now; that's uncanny. To have a crop of former players be as good as the current team must happen quite rarely.
I'm not quite sure which roster and pitching staff I would take. The current Cardinals boast more depth in the rotation, but the former Cardinals have a better one-two punch with Michael Wacha and Jack Flaherty.
Meanwhile, the current Cardinals' lineup has significantly more balance throughout it. While players like Marcell Ozuna and Tyler O'Neill would be more formidable than any middle-of-the-lineup bats the Cardinals have, the lows of Carson Kelly and Harrison Bader make for an easy bottom-of-the-lineup to blow through.
From a bench perspective, the current Cardinals could employ Ivan Herrera, Jordan Walker, Luken Baker, and Pedro Pages The former Cardinals' bench would have to be carried by Tommy Edman -- who has been having a great second half with Los Angeles but didn't qualify for any one position -- Tommy Pham, Patrick Wisdom, and Andrew Knizner to have a viable bench.
The Cardinals' current relievers outpace any collection of former relievers who could be compiled. Thanks to a more balanced rotation, a deeper bench, and a stronger bullpen, the current team of Cardinals would surely beat the former Cardinals in a series.