I don't think you understand the whole pending Free Agent thing, hence why Canucks fans really scoff at this proposal. Giving our best defensemen for a player that will leave, yes, that is definitely great value.
Sure, obviously if you don't think you can sign him then it doesn't make sense.
For Boston they are trading 7 years of Marchand for Tanev, because Boston believes they should be pretty easily able to resign Brad.
Honestly, I think this is just one of those cases where a deal doesn't work for either team. Boston has no interest in dealing it's best winger, and it seems like Vancouver has no interest in dealing their best defenseman.
Boston does not get better by dealing Marchand for Tanev
Vancouver does not get better by dealing Tanev for Marchand