I think this is a great point. The author seems to have a selection bias. All the “great” problems in maths are the ones that have remained hard to solve.
All the stuff in the middle has been solved and then taught and is no longer “interesting”. Or, are we build on those results the new problems are a bit further from the fundamentals so you have to look in specific more specialized domains to find new areas.
What the author seems to forget is that most of the stuff we take for granted now were at one point the cutting edge of maths and obscure to all but the leading mathematicians of the time.
All the stuff in the middle has been solved and then taught and is no longer “interesting”. Or, are we build on those results the new problems are a bit further from the fundamentals so you have to look in specific more specialized domains to find new areas.
What the author seems to forget is that most of the stuff we take for granted now were at one point the cutting edge of maths and obscure to all but the leading mathematicians of the time.