people have been playthrouging minecraft for years, and that has no end too.
It does in a sense that there are a limited number of non-sandbox things to do. For example making a diamond pickaxe, reaching and exploring a section of Nether, defeating the Nether Dragon boss, crafting best gear, building resource generators etc. Sure the game is open ended but it does follow progress until an open-ended stage.
I think the main difference in classification is how the game plays out. A playthrough has some form of scripted progress that the player can move through. This can be advancing stages, completing sandbox objectives or running through a story mode etc. Competitive games like LoL, HotS, HoN, DotA2 etc do not have this form of progress since you are placed against semi-random opponents who are equally as strong as you to "win" or "lose". The only progress made is short-term and really specific to what is going on. I think a more appropriate term would be "matches", "rounds" or any such competitive term.
Playthroughs -> Games where the player progresses and is meant to always "win" or achieve some objective. A good end of such a series is after a final boss or once everything interesting is done that can be done.
Matches -> Games where the player fights other players and is meant to win only half the time. A good end for the series is when the player gets bored of playing.
From a viewer perspective you should always expect a playthrough to go well by the end (player wins, everything gets done etc) even if mistakes happen at times (deaths, restarts etc). Many playthroughs are "edited" specially to remove bad mistakes such as deaths, losses or navigation blunders to improve viewer experience. Matches on the other hand can be expected to have a 50:50 win/loss rate due to the mechanics of the game. People will watch matches regardless of if it is a win or loss since the experience is the progress to the conclusion. Additionally a playthrough usually contains heavy "spoilers" due to the nature of it, where as matches pretty much have nothing to spoil (there are no secrets).
