From what I understand, your main claim was that Beldini should know the issue is solvable in 4-10 years.
And (as a secondary claim not worth retreading) that even if it wasn't solvable in that time, he should somehow be happy to get 10 years' profit.
So unless I've missed something, let's get back to the main point here:
We are not talking about whether the issue is solvable or not, but whether Beldini knows.
At the time, I asked you where you were pulling your 4-10 year estimate from, and how exactly should Beldini be able to come to the same conclusion.
This isn't Earth, this isn't a matter of
technological progress, but a world of
classes and
magic and the progress of the
crafter class and associated skills in particular.
We, as readers, do not know enough to assume the two will be comparable and will both hold to this same 10 year rule of thumb. In fact the story so far has tried to give us the vague impression that developing your skills as a crafter is hard, and since there's little incentive to do so things may have plateau'd as a result, such that in this world Miyabi can be some grand exception.
Beldini, as a character in universe, cannot draw from Earth as a frame of reference.
All he's got is extensive experience commissioning architectural projects from crafters over the years, and seeing how they fare. In fact, I trust he knows better than us how much progress (or lack thereof) crafting has seen over time.
It should be clear from his request that he does not expect crafters to reach Miyabi's
best in 50 years. Not at the current pace.
If there are any specialists at all, it is clear they not common enough an asset that Beldini thinks he'll be able to reliably acquire enough off for it to be a feasible solution.
So:
Unless he asks Miyabi to hold back, Beldini does not know, can not know, nor even
guess as to whether it'll even be possible to have new jobs to maintain, improve upon or replace Miyabi's train
at all in the first place.
And the only guarantee Miyabi will really have held back enough is to get someone else to do the crafting and force him to design around such a limitation.
Sure, we as readers know Karon will pull through, and all crafters will catch up to train-making levels in 6 month's time and everyone on the bus will stand up and clap because that's how these stories go and how we expect things to turn out. But Beldini is a character, not a reader, and so he acts with the limited knowledge he has and is perfectly justified in being the "asshole" that must put his foot down and request that Miyabi pulls his head out of the clouds.
And onto your latest point, that there may be easy improvements to be had?
It is irrelevant: Beldini has little to engineering background and is learning about trains for the first time. He won't be coming up with such improvements nor can he blindly hope there'll be such improvements in the first place. Nor is it his responsibility to think up of those - It is that of those who claim the project is feasible.