To me, the open question is if the novelty factor will wear off. I don't think I would ever get the same person a second Cameo. I'm not even sure if I would keep buying Cameos if the service was well-known. A big part of the gag is "How did you pull that off?!" Maybe there are enough birthdays that it just doesn't matter.
Given enough time, assume you’ll be able to run this on a phone eventually.
You can already happily Photoshop a hugely convincing signed celebrity photo, I don't see people who want such things affected by this or feeling the real thing is worth any less. Giving someone a deep fake as a gift or novelty would feel somewhat strange/hollow.
How big really is the market for a celeb sending a custom 10 second video talking about something, and can this entity really control that market? How do they maintain their margins.
Take Social Media for example, it'd seem there doesn't seem to be a massive player that controls how the sponsorship money flows into influencers, and not for a lack of trying.