There are a lot of players who have reached 50 who never turned in their quests before the (initial introduction of star coins), still have quests that gives XP. There will also be players reaching 50 in the future that will have quests in their logs that gives XP.
To me this is a BUG. Quests XP needs to award Star Coins. Currently they do not.
There are many quests prior to level 50 that players get that can't finish or don't finish. (also side quests).
Sophisticated Pillager is an explorer skill that gives Adventure Quests that awards Quest XP if you complete the adventure.
I'm sure also, level 50 players doing the Fairy Tale quest chains are wondering about their 1000 quest XPs.
Adventure Maps such as LOST SKULL and TROPICAL ISLAND, awards quest XP along the adventure when you complete certain tasks.
All these Quest XP do add up significantly. And they need to award Star Coins.