I've always had a weird soft spot for Plekanec but... better than Brassard? In uh... 2018? Or do you just mean better value relative to the price to acquire him?
Is he a better player then Brassard? No, not even close. But when you start to look at all the changes that would be required to fit in his cap hit and the fact that Brassard doesn't PK... Pleks looks to be a much smarter option.
The issue with Sheahan as the 3C isn't his production... he's getting 33% zone starts and is on pace for ~12g/36pts, almost all at ES. For a 3C who's very good defensively, that's more than fine. The issue is that his fit with Kessel is less than ideal. Kessel thinks offense and RS thinks defense. It's not the end of the world, and I think a different LW then Guentzel would go a long way... but someone like Pleks who can probably be the balance between defense/offense would work just fine. Won't be as dynamic as Brassard... but will allow us to keep almost every piece and should still get the job done.
For Brassard: Sheary+Hunwick out (there's potentially other ways, but realistically, that's it). This also means that Rowney (who Pen fans hate) will almost certainly be in the lineup to handle 2PK C duties.
For Pleks, one of Sheary/Hunwick out (likely the latter). And you're set. We do not lose any real depth, we do not have to worry about having a shit player in just for special teams, we have the players we can role 4 lines... and we keep all our prospects.
I get it... Brassard is the big sexy name. But he specifically (or a player of his caliber) isn't a need. Hell even Pleks isn't a need. The need is a better 4C then Rowney who can win draws and PK. Anything after that is a bonus. But for me, I'd be going after Pleks every day of the week given what Brassard will cost (both directly and indirectly for cap space).