There is an ebb and flow to my game play build in part on time in RL. Some days or weeks I have lots of time and will clock big hours. Other times I will go days without even logging on. Now I don't have hundreds of alts. In fact I have a grand total of 17 level 50's and 17 more ranging from the mid 20's to the high forties. A few were when new powers came out (Seismic Blast, Symphonic Control, Storm Blast). Others were because I liked DP and tried several other combos of that. A few were rebuilds of characters from Live.
I do not farm. if that works for you, awesome, I am not trying to start a thing. But I don't so that means I am able to gradually level up due to the aforementioned not on all the time thing. Once there as others have said I can find more than enough ways to be entertained. Only two 50's are full-on Tier 4 Incarnates, the rest are all over the map. I have One Badge Hunter but also try to work the general content badges on all of the as time passes. Recently it was a two week binge of soloing all of the 50's through the Maria Jenkins Arcs and Tinas "Other Praetorian badge need" missions for Portal Doodle. Others do Flashbacks thematically, one magic another tech.
Then of course there is the team action, with the Nukems or with a PUG or speed runs with Blapperella. There is more than enough content to keep me from being burned out.
And then I start writing stories; six books totaling more than 600 pages per book based on my Munitions Mistress and her evolution which includes those characters I play in game.
Its why I will never find myself in a position where I cannot think of anything more to do.