In any type of 2-for-2 deal where you have the same "positions" swapping... you have to ask yourself -- are you getting an upgrade in at least 1 of the areas? and assuming the answer to that is yes, does the quantum of the downgrade make sense?
Question 1 -- Is Conor Garland an upgrade on William Nylander? To be, the answer is no. While Garland is having a great season, Nylander is much more proven; having scored 20+ goals in 3/4 seasons, pacing at over 35 goals last year, and pacing around 30 this year.
Question 1a -- Is Darcy Kuemper an upgrade over Freddy Andersen? At this point, I would say the fair answer is yes; even though he's only played more than 31 games in a season once in his career. While having the bonus year on his contract helps, it does create a challenge in that he or Jack Campbell would have to be exposed in the ED.
Question 2 -- Is making the upgrade from Kuemper to Andersen "worth" doing the Garland-Nylander swap? and to me, that answer is definitively, no. The implications of a Nylander-Garland swap are far greater than the Andersen-Kuemper swap.
edit:
All that being said, I will give kudos to the OP. From a fan-POV, Garland should be able to come in and give the Leafs a similar level of production to what they're getting out of William Nylander; with perhaps a little more feistyness; and likely a lower cap hit than Nylander's $7m.
If you can re-up Garland at ~$5m on a 3-year deal, it gives the team an additional $2m to work with; on top of the $500k in savings on Kuemper. If maybe you can get the Yotes to retain on Kuemper, even better (with the caveat being the expansion draft challenge).
While I suspect Dubas might be ready to "pull the plug" on Andersen given that it's becoming apparent he isn't in the long term plans, I just don't think he's prepared to entertain touching the big 4. The team seems to be too good to justify making a core shakeup.