In a company of that size, there's literally nobody who would be receptive to such an argument.
End users don't really matter, because they're not in charge of these decisions.
Their managers have a bit more clout, but they're almost certainly not in charge of the vertical that writes the enterprise apps, so they're still not really in charge of these decisions.
Developers writing in-house apps are swamped, both by incoming requests and by paperwork and suchlike. There just aren't enough of them to be able to spare the resources to worry too deeply about UX. Besides, none of them is actually a UX person.
Their managers have limited budget, and can't afford to be allocating resources toward UX. They're worried about their own bottom line, and increased costs due to poor UX invariably come out of someone else's budget.
Whoever's in charge of in-house development cares about these things in only the most abstract of ways. They definitely don't care about individual apps and projects; they're just allocating the resources they get. Which aren't many, because in-house development is a cost center.
The executives in the C suite probably exclusively hold the power to re-allocate budget toward a more optimal solution, but they really, really really don't care about these things. As they shouldn't - the actual dollar cost associated with any inefficiency due to sub-optimal in-house app UX is probably way too small to be worth their attention.