That's why there is a "soft" cap at level 50. I'm at level 47 as a dual-wielding/archery assassin, and I haven't been grinding or doing magic or anything like that. It's totally possible to play the game normally and reach the soft cap using your main skills.
Except that there is no "soft" cap. There is nothing special about 50. In case you were unfamiliar with how the system works:
Each time a skill raises you get experience points equal in number to the new level of the skill. The number of experience points required to reach the next level is current level * 25 + 75. So for example, getting from level 1 to level 2 requires 100 XP (1 * 25 + 75), and getting from level 25 to 26 is 700, 50 to 51 is 1325, and so on. It's the same formula from 1 to 2 as it is from 20 to 21, 50 to 51, and 75 to 76.
There can't be any soft cap, because not everyone plays the same- using the same number of skills. A thief might use Light Armor, Sneak, Lockpicking, Pickpocket, archery, illusion, and one handed, while using nothing else. He may also decide to use all 3 crafting skills. Once he is "maxed out", which would be at a fairly high level given all the skills he is concurrently using, he'd have to start using skills that aren't part of his play style to continue leveling. A warrior might have heavy armor, two handed, block and archery, and be "soft" capped at a much lower level, almost certainly before level 50.
I'm not sure where the idea of a soft cap at 50 came from- perhaps it's close to "average" for most play styles, but more likely, someone just made it up, and other people started believing it. However, it has absolutely no basis in the actual mechanics of the leveling system.
My original statement still stands: It's very likely that you won't NEED to continue leveling in order to "beat" the game in your chosen play styles- unless of course your chosen play style utilizes very few skills, which I'm sure some people's do- but there are some people that have as one their goals to reach max level. They don't WANT to power-level to do so, they want to get there "honestly", but with the way the system is set up now, they pretty much have to power level in order to actually achieve that. There is no way to do so without either power leveling certain skills, OR at some point (more than once) completely changing your play style from warrior to mage to thief, though not necessarily in that order.