I guess that thinking Tibia as a sandbox MMORPG is a much more fair way to start judging the game.It is not supposed to be a literal "xp accumulation race", although most of the gamers tend to visualize the game as that nowadays. So the game shouldn't be only measured according to what is the best way to amass experience and hoard tons of gold. It is also a community and a RPG place to do whatever we want.
Although I think that tibia applied early on an amazing policy of admitting free accounts (something that is really cool. Many oldschools MMORPG do not offer this possibility) they need to make the Free Account more appealing and suitable for people interested on a retro MMORPG. I can tell that playing with Sorcerer or Druid as a Free Accounts offers very little of what it should be in reality compared to what you will see as a Premium (maybe in the past it was ok to be a druid or sorcerer without exori and hunting with swords, but I don't see it on that way anymore), not to mention other basic amenities (haste, magic rope, etc) that would make the game way much more playable and meaningful to newcomers. Saying that, Tibia already gets points for me for allowing people play for free without any trial restriction or such. You can technically hunt at EVERY range possible of level on the Free Area, and even getting to hunt Lost Dwarven or Warlocks on a high level range is a possibility even without spending any cent on Tibia.
However, the game with total playability can only be achieved with Premium Account (which is extremely reasonable and fair). You basically need to pay a constant signature in order to keep houses, quest outfits and of course, the access to much more challenging places. On that sense, Tibia is basically a game where you need to pay really often in order to have access to the most recent features. Just like any company, making profits is a necessity. I guess that it can be considered a solid example of unquestionable Pay-to-Play-Additional-Content, because you could still be someone as a Free Account and have access to the enormous Mainland continent, World Quests, several Bosses, participation in Wars or Guilds, not to mention that you aren't blocked from the game if you can't pay anymore for it, although a huge part of the game and the chance to access all the possibilities of your vocation is something that you can only have as a Premium, which is unquestionable a fair and logic to do if you like to support the game and play it in a more continuous and intense way, with more Bosses, hunts, challenges and quests to be made.
Said that, we should remember about the introduction of Tibia Coins on the game. They were initially a choice available to get more cosmetic stuff (mounts and outfits), which was ok and accepted by the majority of the community without any question. When there was the addition of potions along with blesses and even Kegs/Casks of potions to replenish your supplies, people started to question about if Tibia is now transforming on some sort of Pay-to-Win game. When you start on a brand new server launched now (just like the last ones released), you will basically have the urge to get a house along with a cask and keg ASAP, because you want to get on the lead of the race early on, and this "extra" feature is something that started to be the rule to be followed to competitive players. This is something that Cipsoft implemented and received a receptive demand for it. That's a clear example of a possibility of game objective (the competitive aspect on that case, which is more popular and recognizable on our community) being a prime example of only viable if you are willing to invest extra resources, no matter how good you are with the game mechanics.
Although there are many people happy with things going this way, I would like to explore the monetization of the game in a better perspective. The game can make money with other forms of products. For example, we can have the possibility to sell accesses (permanent or temporary) to Edron by Tibia Coins to a certain Character, not to mention "magic colored powders" that would make possible to apply colors or lights to weapons with imbuements that we want to differentiate from others, just to name a few examples. Cipsoft should explore additional forms of getting money in order to sustain itself in a sustainable manner, but at the same time should preserve the game and steer away from a Pay-to-Win settings, where it only matters on how much are you willing pay instead of being a decent player.
And after that, we could have a more diverse possibility of Server Styles where the player ability gets highlighted above all other factors. For example, we could have a limited number of servers where we don't have any XP voucher or cask/keg, making it a perfect server for people that do not like these features. Anyway, these are my thoughts to your question. Have a good day!