Oh, nonsense. Proxy baptism doesn't make you a member. That's not what they believe. Century after century Catholics taught that if your loved one had the misfortune to die before baptism, no hope. Eternal misery, because Christ said baptism was mandatory. Mormons say, well Christ was right of course but certainly not cruel, so the good news (gospel) is that if you don't get baptized before death for whatever reason, someone will take care of that formality for you so you can still choose in the next life whether you want it or not. If you decide you want it, then you were officially baptized. If you don't want it, you weren't baptized. (Someone else was, offered it to you, and you declined.)

If buying your ticket for a party that requires a ticket but you don't have to go to if you don't want to is "incredibly rude and conceited", you would have good reason to condemn all of Christianity similarly for Christ's claim that he paid the mandatory price for your sins, buying you a ticket to heaven that you'll need (Christians claim) but that you can either accept or reject, as you wish.

Mormons offering to extend the deadline for you to decide into the next life is hardly "incredibly rude and conceited" IMO.