It's easier if you don't draw unnecessary and illogical lines to start with, which is what the dollar threshold is.
> Is Viagra covered, for example?
Apparently, if you buy enough relative to your income.
> This is one of the reasons there's a threshold — to make sure the deduction is only being granted where there are non-routine expenditures.
Which is both a dumb idea and not done well by a share of income rule.
> The tax code views some level of medical expenditures as routine, and therefore non-deductible.
Routine expenditures being non-deductible isn't an intelligent rule; if it is a kind of expenditure that should be deductible, being routine doesn't really change that. If it is a routine expense that is assumed covered by the standard deduction, then you just require itemization to take a specific deduction for the purpose; we don't need special handling for something for which we already have a better and more general solution.
It's also hard to draw lines. Is Viagra covered, for example? This is one of the reasons there's a threshold — to make sure the deduction is only being granted where there are non-routine expenditures. The tax code views some level of medical expenditures as routine, and therefore non-deductible.