While a subscription may seem illogical to a newcomer to Mortal (that's how I first saw it, more than 8 years ago while I was busy collecting undead heads on my Thur-Khur for the first time), it starts to shine only once you have played for a while and have actually managed to comprehend the many intricacies that the game and its community have to offer.
First, you have to look at what this game actually represents. it's a fantasy world, but one much more realistic and much more immersive than most out there. How is that accomplished? The game is a sandbox, it lets the people in the box create their own toys, their own "quests" and their own stories. As a matter of fact, players are the greatest creators and contributors of content in this game. The game is also about player skill, not character skill, so it rewards those who experiment, who invest time and resources into finding the best recipes, the best builds, the best locations to build a base or hunt, etc.
What does this mean? It means that players need to feel that they have a level playing field. That basically anyone is able to start out in this game and improve, with the only requirement for that being their own commitment. Anyone could become the next Master Blacksmith, forging the best greatblades in the realm. Or anyone could become the next great Alchemist, discovering new and improved potion recipes. Anyone could become the next great warchief. Anyone could also try to be a common thief, but only few will succeed to be a Master Thief, like GrayFox - a name that has crossed the boundary of reality and has transcended the realm of legends!
What else do you need in a game where most of the content is created by its own players? You need it to be stable, but you also need it to be reactive to any exploits that players may eventually uncover and start to abuse. You need to be able to take an unbiased approach to any feedback, to test out any imperfections and be even more thorough in order to prevent the remedy from being worse than the original condition.
The subscription fee solves these problems nicely:
1. it provides a level playing field, where the occupation or income level of players outside the game do not affect their success in-game;
2. it provides the developers with a steady income that allows them to continue to tackle difficult mechanics in order to clean them up entirely and proceed with creating deep new systems for more meaningful player content to follow;
3. it discourages developers from spending their limited resources on items that are unnecessary and have a purely cosmetical value, in order for the game to stay afloat.
You also have to take into account the player base. The games mentioned as successful examples of F2P models are (pretty much without exception) mainstream games, widely accepted by a variety of players. Mortal's community, while its active part has both grown and shrunk over the years, and has featured some internal variety, for the most part is a unique subgroup when compared to the general gaming public. Polls in the MO1 forums have shown that median player age is actually significantly higher than that of other MMO's. Also, people in this subgroup generally have a very specific demand, they are looking for this game in particular, so their demand is, in economic terms, relatively inelastic when related to price, i.e. most people in this subgroup would not look elsewhere if faced with a higher subscription price, simply because substitutes for Mortal are hard to come across.
While it can be debated what level of subscription fee is suitable for a game like this, or whether a free trial (e.g. playing for free on the Starter Island, until given player feels ready to embark on their real adventure) can actually help the game in the long run, it is pretty safe to assume, based on the evidence collected so far, that a subscription model is the way to go forward. And this is evidenced by both the developers' insistence on this model, and the community's support for it over the years.
While this model is not without risks (e.g. a low player base that is insufficient to support the continuing development of the game), it would be the developers' responsibility to make meaningful marketing and other expenses in order to obtain a good return on this investment, but from then on, it would be their craftsmanship and the community's own content creation that will drive even more growth.
For us long-time players, I believe that we have proved time and again that we are in favor of this model. And for all serious newcomers out there, I'm pretty sure that they will see the value in this, as well, once they have managed to get their teeth into the universe that is Mortal Online.