Plex is a fine player and I would be totally ok with the Habs signing him back this summer so I'm not trying to diss him. You will not be disappointed with his play especially on the defensive side and his offense is now a bit underrated. He's not as washed up as people have made him out to be.
Plekanec was a pretty good defensive forward with the super low scoring Canadiens. With the high scoring Leafs, he should do very well, as he certainly won't be counted on for any scoring, but will benefit from a higher octane lineup. I can see some pretty good point production in his near future, and his responsible-in-his-own-end play will benefit the Leafs at the same time.
With Matthews, Kadri, Bozak, and Plekanec as the Centres(and Nylander and Marner able to slot in whenever needed.....), the Leafs are very deep down the middle. And with Leivo/Komarov - Plekanec - Kapanen as a 4th line, they can match up against most anyone very well.
I doubt Rychel and Valiev will become much of anything, but the Habs now have four 2nds in the next draft, and there is more to come. That bodes well for the rebuild.
Win-win.