I wouldn't make any claims just yet. Schwarber can walk at a similar rate to Bryant and he's shown far better contact. Across all levels Schwarber has a 14.5% walk rate and a 19.3% k rate. Bryant was at 12.8% and 26.6%. Now to be fair, AAA is probably going to push Schwarber's numbers down slightly. Bryant also had 55 HRs in 773 minor league PAs. Schwarber has 31 in 558 PAs. So, Bryant has a bit of an edge there at a HR ever 14 PAs compared to 18 for Schwarber. But the fact he's even in the conversation with arguably one of the more dominating minor league players over the past 10 years says enough on it's own.
I'm not sure I'd go so far as to say he will be more valuable than Bryant because Bryant's on a 7 WAR pace over 162 games and unless Schwarber is Mike Trout you really don't do much better than that. I would however suggest he may be equally valuable though Schwarber strikes me more as a Rizzo profile than he does Bryant. There's obviously a defensive argument to be made here too. But as I said right after they drafted Schwarber, if you put those walk/k rates together(college at the time) with 25-30 HR power he likely has and compare that to past players you get a rather scary good list of hitters.
Regardless, I wouldn't touch any of Schwarber, Rizzo and Bryant. If you sandwich Rizzo between those 2(R/L/R) you have potentially 3 hitters who could have a .400+ OBP. Last year there were only 3 players in the league with a .400 OBP(Bautista, McCutchen and Victor Martinez) and only two more(Stanton and Werth) above .390. Obviously if you get that out of C you are probably talking multiple time all-star but even if he's only a part time catcher and plays LF the majority of the time you have guys like Manny who have shown what kind of value that sort of bat can have in LF despite being a train wreck defensively. Manny had career 13.6%/18.5% walk/k rates though possibly more power at 17.6 PA per HR obviously at the MLB level vs minors and put up 66.3 career fWAR which is good for 78th all time.
Either way, it may just end up being splitting hairs. I mean as an example does it really matter who was more valuable between Ortiz and Manny in Boston's prime days? If Schwarber is 80% of what Bryant has shown this year thus far then those 3 give the cubs easily the best future heart of the line up of any team I can think of and that's before you start talking about Russell likely bating in front of them, Soler presumably after them...etc. Barring injury, I am not sure how teams are going to outscore these guys in the future. They are 5th in the NL runs/game without all of them being in their prime and without Schwarber, Soler and Castro having a down year/injured. Their average age is 27.1 which is the second lowest in the NL behind Arizona at 27.0 and third in the MLB with houston being youngest.