They last decades, and can be used to repair tens of thousands of items.
To suggest that their environmental impact is higher than that of equivalent favtory equipment needed to produce furniture is delusional.
You are simply reducing manufacturing of one type of furniture with another (that doesn't use the scraps in production) and very much increasing a lot of other manufacturing and increasing the space people need to have available to do these things.
Edit: I'll mention that many folks are simply going to buy the cheapest tools to fix stuff: A decent drill isn't cheap, but a cheap drill might get you by if you only use it lightly and occasionally. If people are using them more, I suspect more will be thrown away. At least the furniture is using some renewable products: Drills aren't made of wood and pressure and glues.
Western society has become full of contradiction...
Consumerism is environmentalism,
War is peace
Freedom is slavery
Ignorance is strength
If you want to fix furniture, you need space and tools, so you need bigger houses (or the ability to move the furniture to a community spot) and tools. Tools will generally be made of metal, and needs factories to make these tools. Some of the parts are available freely: I'm guessing not all are.
Manufacturing will need to be different as well if your pieces aren't designed to be repaired in such a way: Who knows if this would be more environmentally friendly.
Not to mention that it is a skill, so a repair might not even repair it but break it further - but now we've made the tools and things on top of the furniture (that didn't even use up scraps of wood, but caused them).