2. Signing Kyrie could make Lakers worse
In order to make the deal work financially, the Lakers would most likely lose some combination of Russell, Hachimura, Walker IV, Malik Beasley, Jarrad Vanderbilt, and Mo Bamba. Those players would either need to be included in the deal or most likely allowed to sign somewhere else.
It's possible the Lakers could retain one or two of either Hachimura, Vanderbilt, Beasley, or Bamba, However, it doesn't seem likely Dallas does that deal without asking for one or more of those guys, especially with their immediate needs for defense and shooting.
If the Mavs decide they don't want Russell, considering Kyrie would be taking his place in the lineup for the Lakers, LA will probably lose him for nothing in return. It would also require them to add more players into the deal to match salaries, further depleting their depth.
The Mavericks hold all the cards in a sign-and-trade. If Kyrie wants his money and to play for another team, the Mavs have to agree to it.
Consequentially, the only players that could be left on the roster would be LeBron James, Anthony Davis, Kyrie Irving, Austin Reaves, Max Christie, Shaquille Harrison, and possibly Rui Hachimura or Malik Beasley.
The Lakers would likely have to build out the rest of the roster around their new "Big 3" with mostly veteran minimum contracts. From a franchise standpoint, would the Lakers really want to bet everything on three players (James, Davis, and Irving) who have missed a combined 307 games over the last three seasons?
Hypothetically, even if everyone stayed healthy, it would be hard for that roster to contend for a title. Among other things, they would be poor defensively and lack the 3-point shooting needed to create the proper spacing to have their offense operate efficiently.
From Kyrie's standpoint, he would be signing what could be his last big contract without a sensible chance at competing for an NBA championship. Regardless of his play on the court, he would also be the third wheel behind both LeBron and Anthony Davis in the eyes of the fans and media.