Offer sheets being rare in a general sense doesn't mean they aren't appropriate in the right circumstances.
Biggest reason they are rare is because the team with control has incentive to work something out, generally to avoid the threat of the offer sheet. If the cap hit makes sense for "insert offer-sheeting team" it usually makes sense for "team with control" who also has the sunk cost fallacy or not, incentive to keep the player they spent their draft capital on and have been developing. The negotiations generally have to stall before an agent would even go around shopping for an offer sheet, and teams have their own resource constraints so they aren't going around kicking the tires on a bunch of hypothetical offer sheet candidates, when most times it just gets worked out anyways.
Reason number two (of lesser significance) is that you have to give up draft compensation if the offer sheet is accepted. This makes it something more akin to a trade. Since again, you have to be offering a contract the original team is for whatever reason not willing to accept, this can be a pretty big risk. Teams in rebuild mode are generally not punting away draft picks on speculative players, and teams that figure their pick to be late generally don't have cap space up the wazoo to spend on a speculative player.
Reason number three (also of lesser significance) is the cap hold. The period where the player has signed the offer sheet, but the team has not accepted or rejected (I think one week) is a hold of cap space that you don't know whether it will be accepted or not. The original team will likely drag it out for the maximum length of time because they have little reason not to. This means you are limited in the players that you can sign in the meantime while the cap hold is in effect.
Of course, that's all just generally speaking. In this case, I specifically asked "is he a prime offer sheet candidate?" essentially "are the circumstances unique enough so that this is a rare instance, in a general sense, where an offer sheet makes sense"
It seems to me that there is a serious argument that an offer sheet MIGHT make sense, if the Rangers are cap-strapped, Lafreniere is viewed highly around the League worth draft compensation but is currently in a blocked situation, and if the Rangers feel compelled to try and lowball him to fit him under the Cap.
Whether or not the Rangers would match any potential offer sheet is a different question.