The expectation was that we could beat Boston (and we almost did), but the expectation shouldn't have been that we should beat Boston.
Yes, we'd set a franchise record for points, but that itself was outside of pretty much anyone's expectations before the puck dropped in October (with many people still griping about just about anything they possibly could along the way). Hell, some even cited sophomore slumps, improvement from other teams, and less fortune (as far as development and health were concerned) as reasons we could regress in 2017/18. Don't get me wrong, people were excited, with good reason to be, and improvement was certainly expected by most - Some were definitely open-minded enough to think that a season like that was a possibility - but expecting it? I'm not so sure about that.
To be honest, I disagree with pretty much all of the rest of your post - The personnel on our blueline, if utilized properly, is fully capable of winning a Championship behind our forward group, and any need to balance our talent on the back-end can be solved 1) temporarily, by moving Rielly over there instead of Hainsey, 2) long-term with a stud blue-chipper like Liljegren, or 3) through trade with pieces that may not be in our long-term plans (such as an expiring #2/3D in Gardiner) rather than core players.
We're in 'win now' mode, sure, but also 'win for the next 15+' years mode, and keeping guys like Nylander, and patiently developing the youth and prospects that you already have (as opposed to shipping them out as a first possible solution to fill a hole that isn't nearly as large as public perception makes it out to be) is a huge part of that. If you're looking for asset management, trading William Nylander (or Nazem Kadri) would be a pretty bad start, in my opinion.
Frankly, my sights are set even higher than the current Blackhawks - We already have every single thing in-house that we need to build the greatest dyansty since the '90s/'00s Red Wings. My expectation is that we've been on that path basically since Shanahan came on board, and will continue on that path for over a decade, so I'm not gonna stress myself out with things like, "We need to win a Playoff round this year, or else! No exceptions!" while we're still in the early stages of our core players growing together and developing, both despite (and because of) my belief in their ability to already compete with any team in the League on any given night, or over a 7-game series.
'Win now' - with this roster and with this management staff - does not seem to mean 'Win at all costs', and so I don't think that should be the expectation of the fanbase this year (or any, really).