Well, smells like a troll but it's a well-phrased one! :) So let's try...

> Other life will move in after the culling, right?

Not necessarily. I can't speak for the amazon, where burning down a small patch (traditional agricultural practice for millennia) will leave a hole surrounded by life that will move in, but ecospheres disrupted in the large may break what's supporting that ecosphere.

I read an article about cloud forests being disrupted. A cloud forest gets its humidity from mist, which flows in from the ocean a small (18 inches/50cm?) above the ground. This condenses on anything there, trickles down to the ground, waters the plants. But the plant life had been razed, so there was nothing for the mist to settle on; the soil stayed dry so nothing could grow after. The author of the article was requesting ways to restart that (I made a suggestion, he wasn't too impressed).

Also thin soil can be stabilised by plants so it stays. Kill the plants, the soil blows away. Not sure this is 100% relevant but to some extent it is: https://en.wikipedia.org/wiki/Dust_Bowl

"With insufficient understanding of the ecology of the plains, farmers had conducted extensive deep plowing of the virgin topsoil of the Great Plains during the previous decade; this had displaced the native, deep-rooted grasses that normally trapped soil and moisture even during periods of drought and high winds. The rapid mechanization of farm equipment, especially small gasoline tractors, and widespread use of the combine harvester contributed to farmers' decisions to convert arid grassland (much of which received no more than 10 inches (~250 mm) of precipitation per year) to cultivated cropland.[4] During the drought of the 1930s, the unanchored soil turned to dust, which the prevailing winds blew away"

Related to that I've heard of similar happening, poss. in the amazon, where a thin skin of soil is left in some areas, and gets washed away when the plants are not there to fix it - but I can't provide a reference, so treat that as hearsay.

Disruption can also be astonishingly subtle. This does not answer your question but FYI!: https://www.environmental-research.ox.ac.uk/long-shadow-mega... to summarise, now-extinct megafauna are thought to have moved lots of phosphorous around in their poo, benefitting amazonian plants ("For [the plants] the only input of essential phosphorus comes from dust blown from the Sahara and from the dung of animals that had foraged on the fertilised flood plain."). Humans killed them starting ~12,000 years ago, consequence:

"Nonetheless, the huge amounts of phosphorus that were moved by the collective action of the megafauna has left a long legacy and their calculations suggest that the Amazon is only 2/3 of the way through a transition towards a low nutrient state. Today’s trees are still benefiting from the actions of the prehistoric beasts, but as time is progressing the forests are losing fertility."

> 's cool and all, but why wouldn't we expect a [medical breakthroughs from rainforest specimens] to happen with the life that moves in after the hypothetical clear cutting of the Amazon?

AFAUI, no. Varied biochemicals are what produces these breakthroughs, which is linked to varied plant life. If that gets wiped out then it will take millions of years for that diversity to re-emerge, so except in the longest timescale, it's gone. However I am not an expert on this. (But frankly do us humans have to be such tosspots towards anything that doesn't immediately benefit us?[0] These things are jewels in their own right, why can't we value them for what they are)

> I pretty much always get downvoted for these posts where I bounce heterodox ideas of y'all. And it's totally worth it. I'd like to understand why I don't understand the biodiversity concern.

Well, HTH?

[0](Edit: this summarises it nicely https://i.pinimg.com/736x/18/bd/4d/18bd4d5fe4dc42f7a0b01f4c7...)