Quoting: Claesson4Norris
Sure, you could make the argument that they would have been better served taking a forward 5th last year, and take a D this year, but you can't really plan moves out like that. They decided that the most pressing long-term concern was defensive stability, and they had the opportunity to select a guy who projects as an elite level defender and an incredible transition D. He fit the bill of what they really needed, and they didn't want to gamble on someone maybe being available in a year, especially when they weren't planning to be this bad this year.
If they get better goaltending, especially in the first month or so they could have been where Calgary is in the standings this year. The Sens have mostly been competitive aside from that first 15 game stretch. It's much harder to fill the role Sanderson projects to have with a mid round pick. Now the opportunity is still there for then to add another forward they could use, or they can still build further on their Blueline. Having lefties playing the right side isn't that big a deal, especially if it's two elite D like Chabot-Power. They could even trade some assets, like Brannstrom perhaps to acquire an impactful forward to balance things out.
I don't inherently disagree with most of your second paragraph beyond the idea of "if if's and but's were candy and nuts...". A lot of literature pointed at the Sens having misappropriated assets at Murray, but accounting for the injuries is beyond any mortal. I will point out however, that quality defenders are more frequently found in the mid-rounds of drafts than forwards are with the same selections. The quality there exists, and it's usually because specific, fixable elements of defenders are scrutinized much more than their forward contemporaries. Off the top of my head, Edmonton found Jones, Bear, and Marino with very late picks in the 2015 draft, which was supposed to see a lot of talent identified and capitalized on within the first 100 picks. Top-pair guys? Most likely not, but are there more than 15 bona-fide #1 defenders in the league today? Top-four guys for sure though.
I think the Senators should have realized they were likely destined for a top-10 pick this year, which was known to be heavy on blueliners as of two years ago. I don't think an ever statistically-declining Murray, what was left of Dadonov, and an 18 year-old Tim Stuetzle was near enough to declare the team remotely competitive, especially in what was then only going to be the Atlantic Division. The Sabres' drop off wasn't expected by most anyone, so at best they were hoping to be better than Detroit and in the running for 5th with Buffalo and Montreal. I think any expectations of keeping pace with Boston, Florida, Tampa Bay, or Toronto over the course of any length of season is very unrealistic.
I do support the notion of thinking a bit more "big-brained" and taking Sanderson last year though (again, money now is worth more than money tomorrow). I liked the pick, even if BPA trended more towards Raymond.