Are you talking about cheap drones replacing modern fighters, or cheap drones shooting down modern fighters?
They will absolutely replace modern fighters to a degree and already have to an extent, like we've seen in Ukraine with drones serving as close infantry support etc. instead of manned aircraft.
There are still some things "cheap" drones won't be able to do.
Specifically, if you want to carry big bombs and missiles, you need a large aircraft even if it's unmanned. The drone is also presumably going to need to be able to survive enemy air defenses to some extent. So you wind up with a big expensive drone like the MQ-9 Reaper ($30mil plus, lol) that is approaching the size and complexity of a manned fighter.
Also, and I'm not sure if this is what you meant, but "small, cheap drones" are not likely to be shooting down modern fighters in the way that small cheap drones are currently killing tanks directly.
If you want an aircraft that can fly at mach 1.6+ and shoot missiles at other things (some of them stealthy) traveling at mach 1.6+, you are going to wind up with something fairly close to the size/cost/complexity of an F-35 even if you don't have a human being sitting inside.
Alternatively if/when starship is up and running it should be pretty trivial to ferry a lot of JDAM-esque weapons up to an orbital launch platform. From there you could practically throw bombs down in a suborbital path to a target and let the JDAM fins guide it in through the atmosphere.
A starship already costs around 90 million USD at cost (estimated based on parts cost currently) and is intended to cost around 10 million USD per launch commercially if it's able to be reused regularly.
And a single launch could carry a lot of bombs (like 50-400 based on the size). So on the high end after factoring in the cost of the bombs, amortising the cost of the initial orbital platform, and assuming a higher cost of like 120 million USD for a single launch, that would still almost certainly come out cheaper than sacrificing an MQ-9 or a much more expensive missile. And of course if you can get that cost down to 10 million USD per launch then other than artillery, that's going to be the cheapest way to deliver ordinance to any location in the world by far (after factoring in the cost of flight time for jets, etc).
They will absolutely replace modern fighters to a degree and already have to an extent, like we've seen in Ukraine with drones serving as close infantry support etc. instead of manned aircraft.
There are still some things "cheap" drones won't be able to do.
Specifically, if you want to carry big bombs and missiles, you need a large aircraft even if it's unmanned. The drone is also presumably going to need to be able to survive enemy air defenses to some extent. So you wind up with a big expensive drone like the MQ-9 Reaper ($30mil plus, lol) that is approaching the size and complexity of a manned fighter.
Also, and I'm not sure if this is what you meant, but "small, cheap drones" are not likely to be shooting down modern fighters in the way that small cheap drones are currently killing tanks directly.
If you want an aircraft that can fly at mach 1.6+ and shoot missiles at other things (some of them stealthy) traveling at mach 1.6+, you are going to wind up with something fairly close to the size/cost/complexity of an F-35 even if you don't have a human being sitting inside.