The game-changer here would be bringing cost down and range and accuracy up.

I don't know what the cost of a modern army's soldier in the field is (or how that varies by national average income --- one of the cost drivers for the US military is simply typical wages and income), but as a very naive ballpark estimate, the US Army which boasts north of 450,000 active-duty soldiers and a $173 billion budget (FY2022) is therefor looking at about $380k/soldier-year, total cost.

I'll estimate that actual combat troops are roughly 10% of the total, so we're looking at about $4 million per pair of boots on actual ground.[1] Maybe.

A Tomahawk cruise missile costs about $1--4 million (varying by source). Single use.

A Predator MQ-1 drone costs about $40 million. Multi-use. The Hellfire missile (with which Predators may be equipped) run about $58k -- $150k each.

The DJI consumer-grade drones being used in the Russo-Ukraine war run about $1k--2k each. Multi-use.

Switchblade 300 and 600 drones run from $6k to $70k per unit.

Significant challenges for smaller drones are the warhead that can be carried, range, identification of targets, and avoiding counter-battery fire, in which the launch, recovery, and/or control points for drones are identified and attacked by retaliatory fire. Among other advantages of single-use drones are that such counter-battery fire is more challenging as one cannot simply follow the drone back home.

Slaughterbot-scale drone attacks strike me as reasonably implausible in contemporary warfare. Targets are insufficiently dense, ranges are too short, and costs are comparatively high. A swarm of, say, 10 to 100 drones might be viable, but that's going to only take on 10 to 100 individual targets. Swarms of 1,000s of drones would require a very capable military organisation (of which the US, possibly a few other NATO forces, and perhaps China might be capable of affording such weapons), and still have to be transported relatively close to the field of battle by some other means. Though drop-shipping a standard 40-foot container might well be one option available. Others could be launching from a larger mothership (a drone aircraft itself, manned cargo aircraft, a ship or submarine, or land-transports such as trucks or railcars). Those are easier than launching an manned land-invasion, but still a complex undertaking.

Even states that have significant technical capabilities and relatively low inhibitions (Israel, Saudi Arabia, North Korea, Iran, Russia) seem not to have widely adopted or deployed drone weapons, though whether that's a technical limitation or strategic decision I'm not sure.

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Notes:

1. As with all estimates, this is eliding a tremendous amount of detail and includes much guesswork and outright ignorance. I'd greatly appreciate pointers to accurate information on fighting vs. support troops. I'm omitting national guard though I'm well aware that they serve combat roles. My sources don't give hard numbers on soldiers in infantry, though I'm finding that the 1st, 2nd, 3rd, 11th, and 25th divisions are considered "infantry", that a typical division is 10k -- 25k soldiers, so we're looking at 50k -- 250k soldiers assigned to infantry divisions, though again some fraction of that is support roles. Still, $0.5 -- $5 million for a soldier-year on the ground seems a reasonable ballpark.