That's interesting, I've always personally wondered about this. Thanks for doing it and reporting the results.Unfortunately, this is a myth.
Grind is optional. Farming high XP/min quests is most definitely the most time-efficient way of gaining XP, but this doesn't suit everyone. Luckily you don't need to do this.
To prove this point, for my 40th life, we decided to:
1. do every quest once and once only ... you have no idea how much pain this caused me to leave Shadow Crypt after only one run, but them's the rules
2. do challenges just once to try and get as many stars as possible and the first time XP bonusses that come with them - we skipped kobold island challenges completely because they're a pain without a full party of arcanes to keep the crystals repaired
3. tag each explorer for each wilderness area on a single run, check for rares but not reset and repeat to try and get them all, and just pick up whatever slayers we got in the process
4. not drink any XP pots, not even the ones you get from House Cannith challenges or turning in Eberron Dragonshards
Now both myself and my regular TR buddy have the Greater Learning Tome from the MotU expansion. I'm a VIP, but he isn't so I get the extra 10% XP from that, but he doesn't. As a VIP I get all of the volumes in the Monster Manual but he's missing the latest one I think. So I did pick up more XP from the Monster Manual in the process than he did.
The end result was that we both reached 20 without having to repeat a single quest, just by doing an Elite Streak. We ended up banking levels and ended up having to level to avoid wasting XP, which unfortunately meant we didn't get the Elite Streak bonus for those quests. We skipped the Lords of Dust chain completely and just came back to do it on Epic because the Epic XP is so good. Probably half of the raids we didnt' have a party at-level for so we skipped those too. It's definitely not the fastest life I've ever done, but we proved it's doable. Also, when you don't have an XP Pot burning away, you're much more relaxed about taking your time through the quests, so I'm confident a semi-capable party of 3rd lifers could maintain their elite streak at least until level 18.
So I'm afraid grind is 100% optional. You might be missing an adventure pack but that's something you can fix.