If you want to run a low-magic campaign but still have most D&D character classes be viable (that is, at least as viable as when playing with magical items), there's a simple fix:
Don't change the math. Instead, change the story.
Non-magic weapons without punishing the players
Instead of a weapon being a magically enchanted Longsword +1, why not extend the masterwork idea. Use the rules as written but explain it differently: Maybe there's a famous blacksmith who is able to create weapons far better than usual (+1).
If the party manage to find special materials (rare metals, 'dwarven steel', hide from a rare creature,...), the smith can create even better weapons (+2).
And if they can convince an alchemist (chemist) to teach them the process of hardening the dwarven steel using chemicals extracted from a plant that only grows in a specific area in the mountains, or they meet a traveler from a far away land that can teach the smith the art of folding metal (similar to certain japanese or arab swords)...
Getting the 'improved' weapons should probably happen earlier than in a normal campaign because you will still lack the usual buffs and enhancements to strength and dex.
Making up for the lack of ability enhancement items
I would advise you to use a high point buy to begin with, so STR/DEX are higher in order for the characters to hit.
Later in the story, you can add opportunities to 'train' their main stat at a suitable time, or allow them ability increases on additional levels.
What about healing?
Using the two methods above, you can create a non-magical atmosphere without having to modify all the monsters to account for the changed math. Of course, healing is still a problem, but this could be solved by allowing alchemical items to make up for some of the common healing magic.