This is all classic Borg. Every move. This is about squeezing value out of our situation from every possible angle.
Let's say we get Burns. Let's say San Jose pays us a 2nd and a 3rd to take him at 50% retained (I have no idea what the actual trade would be, hence "let's say"). Well, now, instead of re-signing TDA, we have Burns in the lineup (a slight downtick or uptick in on-ice value depending on your opinion), but now we have 2 extra 2nds, 2 extra 3rds, and an extra 4th compared to simply signing TDA. Boom, value.
The Borg does things very differently in a lot of ways, but one of the things they have a firmer grasp on than most of the league is the notion that in the NHL, a player's value is inextricably tied to his cap hit and contract situation, in more ways than one. And they use that fact to move on from good players, bring in certain players, take advantage of teams who get themselves into trouble by not having as firm a grasp on that fact (e.g. Marleau, KK), etc. Now, if they are kicking the tires on Burns, it's probably because they believe he's a good player still (their opinion on how well he'll age, who knows). But you can't say it makes no sense to move on from TDA to Burns if the asset price is right, assuming they believe Burns can still be a player.