Now this one is a can of worms as there is more than one answer here.
W are dealing both with casting skills and fishing skills, all-be it they are closely related to some extent.
Bottom line is l could not agree more with you.
Unless the rod is a total piece of junk , and even then a good caster would be able to deal with it is down to the angler ability.
No question of doubt that casting skill an make the world of difference so far as your catch returns, and further allows for the angler to fish many different techniques.
Its not so much a issue of better to spend money on casting lessons rather than the rod assuming that the angler has a well matched rod and line outfit.
Saying that here is my view, and bear in mind l do deal with over 500 fly fishers a year both at my schools and guide days.
Its not a question of being able to cast 100ft as few fly fishers can do that to start with.
It is being able to make good average range casts from 10 to 50ft in the case of trout.
If you wish to make 100ft casts then you have to have very good basic casting skills and then build on those which will take many hours of practice.
No doubt that a entry level fly fisherman would benefit from a series of fly casting lessons.
Unless there after that individual perseveres to practice then almost always very bad inherent faults will develop, which are to a point very difficult to at a later point in time be rid of.
We then have the further issue of knowing how to use those basic skills when fishing, which as we know differs.
Casting large streamers is a very different deal to delicate dry fly presentations, all be it both have one factor in common which is knowing how to load the rod with minimum false casts, which is one of the major faults, excessive false casting, which is down to 3 main reasons.
Insufficient fly line out of the rod tip to load the rod or a line weight not matched to the rod action, or both.
Line weight not matched to rod is a common problem l see. There are many so called 5wt rods that are best suited to a 6wt and not a 5, here again there may be another issue and that is choice of line profile.
Both are likely to cause problems for a angler who is not able to recognise the problem.
High end rods will not turn bad casting techniques into a good ones, that's a fact. On the other hand a high end rod well matched with the right line will allow the angler if he works at good casting skill to be a good caster. Which may well from that point on allow the angler to match good casting skill with given fishing situations.
They both go hand in hand, they have to be a blend of skill levels.
DW