The main advantage of being among the first to board the ship is being among the first in line for dining changes and dining/activity reservations that were unavailable online. Since Platinum cruisers are able to book cruises and dining/activity reservations earlier than other groups, they are actually less likely to need this advantage. I can see why from Disney's perspective, it's better to give people who couldn't book things online a better shot at being able to get them once onboard. That gives these people a better cruise experience, and makes them more likely to sail again. If the only way to get popular reservations is to have sailed at least 10 previous times, it is discouraging for those who have sailed just once or twice.
I am a Platinum cruiser. If I can't get a Palo, Remy, Enchante, character meet, or alcohol tasting reservation, I'll be disappointed, but not crushed, because I've already done anything I wanted at least once on a previous cruise. If I were about to go on my 3rd or 4th cruise and still hadn't been able to get some of those because Golds and Platinums both book early and board early, I would feel like there's no hope and am more likely to look at another cruise line where I can get an adult dinner (especially if I'm cruising with kids who are getting older).
The core Disney market is families with children, who may have limited vacation availability and may not be able to cruise 10 times before they feel the kids are "too old." Disney wants to get these families to cruise several times, spending money on souveniers and developing potential interest in other Disney products, such as movies and parks, but realizes that they may never make it to Platinum. The markets for most of the other cruise lines trend older than Disney's, with people who may cruise on the line for years to come if they're happy. Even the ships that attract younger families are part of fleets that have ships skewing older, so it makes sense to give major incentives to keep cruising on the same line. DCL knows that many of their passengers cruise because of kids. Obviously there are many passengers who take Disney cruises without young kids, but we are not the audience
Disney Cruise has decided to target, so giving us additional loyalty rewards isn't important to them.
According to the posted article, Carnival also allows top-tier guests to drop off luggage at their rooms upon boarding. That would make early boarding much more appealing to me, but wouldn't be enough to influence my choice of cruise lines. I can see why it might dissuade a top-tier Carnival cruiser from looking at Royal Caribbean, though.
