Also he did not in fact teach all runesmiths since that is not how teaching works, he invented the discipline they make use of. It is the same reason everyone to learns basic geometry is not a student of Pythagoras.
A fun reminder that it wasn't even Thungni alone that figured out how to teach Runes, but a coordinated effort of Thungni and Grungni. Thungni saw the Glittering Realm and discovered Runes, but only by working with Grungni was it made possible for normal dwarfs to learn them and the ability to use Runes is passed down by
Grungni's lineage not Thungni's.
Thungni is someone who at the end of the day I think cannot really… communicate what he truly understands and so tries to teach via examples, parables, and metaphor.
The Guild of Runesmiths was founded by a man who couldn't teach but could inspire, and who with help translating what he seemingly intuitively could understand into Runelore.
Thungni, I will give some props did work with others to teach them but I imagine he was one of the professors at a college that is a genius and really shouldn't be teaching because their brain is just wired to understand calculus and set theory and so cannot explain well foundational theorems or steps. He can work the greatest wonders at the top of the field, but asking him to show his work is like asking someone to
reinvent the proof that 1+1=2.
Durin I think may have been crushed by that. By the consternation of seeing perfection and being a genius but still always falling short of the example set before him by what surely felt like miles.
So, what happens? Thungni goes the other way. Hands off and set the challenge for those with a mindset to try, be willing, and capable of taking his place without being buried by the weight of legacy.
Was it great? No, it is a complete nightmare of a method but it's yielding results so it wasnt the worst idea.
But the Guild needs someone who can actually be a teacher of teachers and a student of students. Snorri still learns so much from the people he taught and the Runelords he works with, synthesizing together vast arrays of tunes into ever greater heights and inventing ways to allow those less gifted to follow in his footsteps. Snorri has worked relentlessly on Windsight and creating ways for Karstah, and inevitably others in time, to see what he sees and learn what he's learned without needing centuries of trial and error.
The Guild needs someone who ensures that a stairway to success is built, for the Runesmiths of tomorrow to follow. That the body of Runelore and technique can withstand the rest of time rather than be as ephemeral as the life of a single Dwarf.
Taking the position of Heir will involve doing what Thungni wants? Sure, let's advance society by leaps and bounds by both teaching and examples. Let's invent whole new methods of using magic that no one ever had imagined impossible with others' help. Let's ensure that Runelore is only passed on to the worthy, even if the wait to find someone worthy takes longer than the lifetime of the Runelord who discovered it.
Snorri, I think, has the chance as Thungni's Heir to via Durin's line to elevate the Runesmith's from a collection of fully independent thinkers into a collection of a thousand independent gears that collectively turn the wheel of progress forward. Picking up slack where one fails until it is replaced by another in the future.
That's why I want to claim the title of Heir. Because not even Thungni created Runes alone for all he invented the field. And Snorri is the one best able to understand that as the source of the mantle he'd be taking onto his shoulders.