I believe there are very few games out there that offer this very feature, the LACK of an end game. Most mmorpgs struggle to maintain the commitment and attention from their players once they have reached a later stage of the game, and they do so by creating a new scaled measurement system (by item level, power or similar features) while at the "end" of the level journey for the player.
I believe Tibia has a competitive advantage in that field, because that "limitless" horizon is, by all means, true and as direct as it can be. Tibia has always been a competitive game, be it on PVP or on PVE servers, and the prospects of becoming higher level, more skilled or simply richer than other players are what drive most people. After all, it is a 2D pixel game that competes with veteran platforms such as WOW or even ESO, games that offer much higher alternatives in terms of graphic, design and perhaps content.
That doesn't mean that the lack of level cap fills the game with everlasting fun. Grinding is still boring after a certain period of time, and that is a constant across games, despite the leveling system (progressive or not)
CONCLUSION: no, Tibia should NOT let go of one of their main competitive advantages.