This just shows that you don't know what you're talking about. At all. Dallas' biggest issue is goaltending, and it's not the Spezza contract that sunk them there. It's that they have $10.4 million of their cap tied up in two mediocre goalies in Lehtonen and Niemi (the latter's good seasons coming when he played on powerhouses).
There were plenty of reasons to trade Spezza, to be sure. His wonky back, his age, his contract situation, etc.. But if we're being honest, none of those were why he was moved. He was moved because he agreed it was time to move on (and after 11 seasons of what he put up with from fans and the media here, I don't blame him), and to be used as a scape-goat by management for the team missing the playoffs and for MacLean's ego undermining why he had success as a coach in the first place.
The idea we needed to move Spezza to improve the team's defense? I might be able to buy that is MacLean had EVER employed a decent defensive system while he was coach here. But every season under him had the Sens giving up a ton of shots (the difference was in the first two seasons we at least tended to outshoot opponents, and in 2012-13 we also got amazing goaltending from Anderson, Bishop, and Lehner in a shortened season). We were cr@p defensively under MacLean both with and without Spezza. Hell, he gave Greening-Smith-Neil 15 mins or more some nights, and that line was a tire-fire in the defensive zone.
And what happened the first season without Spezza? MacLean got axed early on while the team was struggling with the exact same issues, and it needed a miracle run the likes of which we'll probably never see here again to make the playoffs. And it came with some journeyman goalie who could be out of an NHL job by this summer. What happened the year after that run? Oh, right, same thing, except defensive coverage under Cameron may have actually been worse in some regards.
It's only years later under Boucher that the team has improved defensively (and frankly we're still not great in that regard). So this idea that Spezza was seriously hurting the team defensively doesn't fly. What hurt this team defensively was two head coaches who couldn't implement a defensive system if their lives depended on it.
I'll also take issue with Spezza's "last few seasons" with the Sens being bad. His last season was not good (though he was still putting up decent point totals), but lo and behold, once we actually got him some talent he meshed with in the form of Hemsky, he was pretty good. It also made people forget just how garbage Michalek had been up to that point in the 2013-14 season. 2012-13? Where Spezza played just 5 regular season games due to injury? You're honestly going to judge his season on 5 games? And in 2011-12 he was great, finishing 4th in the league in points, and being a major reason for the team's turnaround along with Karlsson's emergence as a superstar.
So of his last 3 seasons, one was bad (but still had decent point totals), one was a wash, and one was great. Truly, you're narrative is bang on! He'd sucked for years!
Let's face facts, people's obsession with Spezza not being a two-way player is ridiculous. The guy put in tons of effort to improve on the defensive side of the game and it was noticable. Was he ever going to win the Selke, or be like a Datsyuk? No, but if that's your standard then prepare to be disappointed a lot.