This is revisionist thinking. At the time it was a good move and even with hindsight was worthwhile.
It should also be considered that Brown and Zaitsev would have less opportunity to play on the Leafs vs where they are on the Sens depth chart.
"Too much depth" is only a problem in TO.
It's funny I went on a little Dangle binge fest the last few days and of all the takes that aged poorly it was right when the Leafs signed JT and he kept saying "I don't even care who is on the 3rd or 4th line, ghosts could play there".
And yet you look back on all the Cup winning teams and they all got very important production from their depth players. Between Crosby and Malkin the biggest goals in the Pens recent playoff history came from Staal, Talbot, Rust, Kunitz, Hornqvist..
The takes ok "we lost because Marner and Matthews couldn't score" from Leafs fans make me question if they stop watching hockey once the Leafs season is over because then they would know that's what happens to star players come playoff time, the space that was there to create offense very quickly disappears.
I would know I saw it time and time again with Rick Nash who would score 40 goals easy (and on bad offensive teams when scoring was lower, Benn won the Ross with a grand total of 87 points) during the regular season but just couldn't find that space the deeper we got into the postseason.