*** As a serial bloviator, I might as well quote my own bloviations, regarding Levels (XP is a relative mechanic, not really zero-order structure):
quote thus... the inherent flaw in Level Based Game design. (nothing personal CDPR, just not the best choice for 'durable' gameplay)
There should be one level of difficulty higher than Death March -- Skill Based -- Example: If you're asked to fetch some poison/acid from an Arch Griffin, this should not be a fetch quest, to kill that griffin should take a lot of time and preparation, but most importantly -- lots of tactical and strategic skill -- Skill Based. (this assumes that AI uses skill as well -- good AI -- and mob are not just Hit-Point-Pinatas) How long does it take you to kill that Arch Griffin...? Five minutes, three minutes? Took me one minute on DM, on first play-thru because I had already leveled way passed said content simply by doing the primary Story and only the Story. (no mods installed). What should have been a challenge, had expired; would have been a challenge if my pc and mob were -- equally matched, stat-wise
Skill-Based Gameplay requires one very important foundational, structural mechanic -- Level Scaling (or no traditional levels at all -- just talent points per level). Level Scaling simply means: you will never level past a difficult encounter, or a difficulty setting (or that there is no 'vertical' progression, only horizontal). Guild Wars Original did this well (hard mode), Star Citizen will do this eventually (dynamic difficulty scaling), plenty of other examples -- the founder of RPG pen/paper has stated, to paraphrase: were he given another chance he would have never used numbers (stat growth) as the functional foundation for RPG -- it was a mistake. Heroics by Number was the wrong path.
You see, much of the RPG industry is tragically stuck in a cycle of collective stupidity. Following the flawed path of predecessors who got a few things right -- the journey, the quest, the adventure. But seeded a need for pathing as control of content access called -- progress. Progress is simply the stupidest word ever conceived by sentient beings. Progress in Sandbox, in the traditional sense, is impossible. (in other words, progress in real life is an absurd concept) It is why so many games begin to malfunction, mid to late game, and why the WoW swarm consumes, but is never satisfied. (societies and civilizations do the same thing -- thinking progress is something you can control, nope).
So, if you're not willing to gimp yourself, and just stay in starter gear -- or spend no talent points -- or under play the many encounters -- in other words, not play the game the way it was designed. Then yes, eventually you'll run out of stuff to do.
edit, I've about 160 hours into W3 (2nd play-thru), but I play only on DM, and have turned all the hud-crutches completely off, using only witcher senses. I'm gimping myself in a more --immersive-- way. Just landed on Skellige, still tones of content, because I'm playing for fun, not for progress, not for 'completion'.