Don't forget potions. You could easily make use of muggle civilisation's amazing mass production ability by having them automate every step but the ones that need magical power. The magic drain from potion creation is minimal, so one or a few wizards could effortlessly power a production line.

Muggles also happen to be very good at large-scale farming, which could easily be applied to the production of magical ingredients, even rare ones.

Add in wizards' ability to cast permanent enchantments at no cost, and their vast array of utility spells (Vanishing Charms alone would be a godsend for any factory), and you have undreamt-of mass production potential.

Small population sizes aren't an issue when you have two worlds' worth of force multipliers.

if wizards were public about their abilities, a higher proportion of wizards (even low-powered wizards) in the muggle population would be identified and trained

There's no such thing as a "low-powered wizard", and all wizards in Britain are automatically detected magically (at birth?)

It is implied that in HPMOR there are - presumably third-world? - countries where they "receive no letters of any kind". So potentially a complete breakdown of the masquerade might allow the least sane Muggle governments to track down and kidnap wizarding children for their own use. (I'm a little confused by this, though, since spontaneous untrained magic should be a serious issue if muggleborns aren't being dealt with.)