I think that Alzner contract is pretty optimistic for the Caps, and Oshie slightly so as well.
Most of the prized UFA wingers are cashing in heavily on their term and getting contracts until at least age 35. Eriksson ($6Mx6y, age 31), Lucic ($6Mx7y, age 28), Okposo ($6x7y, age 28), Ladd ($5.5Mx7y, age 30), Cammalleri ($5Mx5y, age 34), and Moulson ($5Mx5y, age 32) for example. Getting a 29 year old Oshie to settle in a shorter deal might be a challenge, knowing that this is almost certainly going to be his last major contract. I'd peg him at $6M for 6 or 7 years on the open market.
As I've said elsewhere, while I love Oshie, I want to see more of him at even-strength before I'd sign him to $36M or $42M contract. Right now he's terrific on the powerplay and a really popular player among fans (and did a lot to erase his previous playoff issues this post-season), but his even-strength production wasn't overwhelming and we've seen lesser players succeed in that powerplay role. I love every penny of the $4.175M we're spending on him now, but with raises at least coming to Kuznetsov, Alzner, and Burakovsky, giving Oshie a raise as well likely mandates the subtraction of players elsewhere on the roster. It could be the right move, but I want to see levels of even-strength production that assure me he's not a replaceable part. That could be increased ES scoring with Ovechkin or continuing a similar level of production as the catalyst on his own line with tougher deployments.
Alzner would have to take a serious hometown discount to stick around at $4.5M. Maybe the stability of 8 years is enticing, but even that's only a $36M contract and eats up almost all of his remaining career. When you look at the AAV that guys like Staal, Girardi, Orpik, Sekera, and Petry received, it's a hard sell to keep Alzner down at $4.5M. $5M seems like the very low end of what he would fetch on the open market, and some trade board posters have hypothesized $6M-$7M offers being thrown his direction. In principle, I'd be wary of overpaying for Alzner. As much as I love him (and own his jersey), I do feel the Caps fanbase as a whole likely overrates him somewhat. Unfortunately, without any near term replacements in the system, the Capitals are pretty much bound to extending him.
Then there also is the concept of leaving a player a UFA at the time of the expansion draft purely to dissuade Vegas from selecting them, only to re-ink them afterwards. It runs the risk of bleeding into the period where players have the option of talking to other teams, so there's risk associated, but it might be worthwhile to essentially provide "soft protection" for a couple of players. Similarly, they may also want to factor post-season performance into their evaluation of these guys (particularly Oshie).